cs8493 - operating systems unit 4

Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Chapter 10: Mass-Storage
Systems

10.2 Silberschatz, Galvin and Gagne ©2013
Chapter 10: Mass-Storage Systems
 Overview of Mass Storage Structure
 Disk Structure
 Disk Attachment
 Disk Scheduling
 Disk Management
 Swap-Space Management
 RAID Structure
 Stable-Storage Implementation

Objectives
 To describe the physical structure of secondary storage devices
and its effects on the uses of the devices
 To explain the performance characteristics of mass-storage
devices
 To evaluate disk scheduling algorithms
 To discuss operating-system services provided for mass storage,
including RAID

Overview of Mass Storage Structure
 Magnetic disks provide bulk of secondary storage of modern computers
 Drives rotate at 60 to 250 times per second
 Transfer rate is rate at which data flow between drive and computer
 Positioning time (random-access time) is time to move disk arm to
desired cylinder (seek time) and time for desired sector to rotate
under the disk head (rotational latency)
 Head crash results from disk head making contact with the disk
surface -- That’s bad
 Disks can be removable
 Drive attached to computer via I/O bus
 Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel,
SCSI, SAS, Firewire
 Host controller in computer uses bus to talk to disk controller built
into drive or storage array

Moving-head Disk Mechanism

Hard Disks
 Platters range from .85” to 14” (historically)
 Commonly 3.5”, 2.5”, and 1.8”
 Range from 30GB to 3TB per drive
 Performance
 Transfer Rate – theoretical – 6 Gb/sec
 Effective Transfer Rate – real –
1Gb/sec
 Seek time from 3ms to 12ms – 9ms
common for desktop drives
 Average seek time measured or
calculated based on 1/3 of tracks
 Latency based on spindle speed
 1 / (RPM / 60) = 60 / RPM
 Average latency = ½ latency
(From Wikipedia)

Hard Disk Performance
 Access Latency = Average access time = average seek time +
average latency
 For fastest disk 3ms + 2ms = 5ms
 For slow disk 9ms + 5.56ms = 14.56ms
 Average I/O time = average access time + (amount to transfer /
transfer rate) + controller overhead
 For example to transfer a 4KB block on a 7200 RPM disk with a
5ms average seek time, 1Gb/sec transfer rate with a .1ms
controller overhead =
 5ms + 4.17ms + 0.1ms + transfer time =
 Transfer time = 4KB / 1Gb/s * 8Gb / GB * 1GB / 10242KB =
32 / (10242) = 0.031 ms
 Average I/O time for 4KB block = 9.27ms + .031ms =
9.301ms

The First Commercial Disk Drive
1956
IBM RAMDAC computer
included the IBM Model
350 disk storage system
5M (7 bit) characters
50 x 24” platters
Access time = < 1 second

Solid-State Disks
 Nonvolatile memory used like a hard drive
 Many technology variations
 Can be more reliable than HDDs
 More expensive per MB
 Maybe have shorter life span
 Less capacity
 But much faster
 Busses can be too slow -> connect directly to PCI for example
 No moving parts, so no seek time or rotational latency

Magnetic Tape
 Was early secondary-storage medium
 Evolved from open spools to cartridges
 Relatively permanent and holds large quantities of data
 Access time slow
 Random access ~1000 times slower than disk
 Mainly used for backup, storage of infrequently-used data,
transfer medium between systems
 Kept in spool and wound or rewound past read-write head
 Once data under head, transfer rates comparable to disk
 140MB/sec and greater
 200GB to 1.5TB typical storage
 Common technologies are LTO-{3,4,5} and T10000

Disk Structure
 Disk drives are addressed as large 1-dimensional arrays of logical
blocks, where the logical block is the smallest unit of transfer
 Low-level formatting creates logical blocks on physical media
 The 1-dimensional array of logical blocks is mapped into the
sectors of the disk sequentially
 Sector 0 is the first sector of the first track on the outermost
cylinder
 Mapping proceeds in order through that track, then the rest of
the tracks in that cylinder, and then through the rest of the
cylinders from outermost to innermost
 Logical to physical address should be easy
 Except for bad sectors
 Non-constant # of sectors per track via constant angular
velocity

Disk Attachment
 Host-attached storage accessed through I/O ports talking to I/O
busses
 SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator
requests operation and SCSI targets perform tasks
 Each target can have up to 8 logical units (disks attached to
device controller)
 FC is high-speed serial architecture
 Can be switched fabric with 24-bit address space – the basis of
storage area networks (SANs) in which many hosts attach to
many storage units
 I/O directed to bus ID, device ID, logical unit (LUN)

Storage Array
 Can just attach disks, or arrays of disks
 Storage Array has controller(s), provides features to attached
host(s)
 Ports to connect hosts to array
 Memory, controlling software (sometimes NVRAM, etc)
 A few to thousands of disks
 RAID, hot spares, hot swap (discussed later)
 Shared storage -> more efficiency
 Features found in some file systems
 Snaphots, clones, thin provisioning, replication,
deduplication, etc

Storage Area Network
 Common in large storage environments
 Multiple hosts attached to multiple storage arrays - flexible

Storage Area Network (Cont.)
 SAN is one or more storage arrays
 Connected to one or more Fibre Channel switches
 Hosts also attach to the switches
 Storage made available via LUN Masking from specific arrays
to specific servers
 Easy to add or remove storage, add new host and allocate it
storage
 Over low-latency Fibre Channel fabric
 Why have separate storage networks and communications
networks?
 Consider iSCSI, FCOE

Network-Attached Storage
 Network-attached storage (NAS) is storage made available over
a network rather than over a local connection (such as a bus)
 Remotely attaching to file systems
 NFS and CIFS are common protocols
 Implemented via remote procedure calls (RPCs) between host
and storage over typically TCP or UDP on IP network
 iSCSI protocol uses IP network to carry the SCSI protocol
 Remotely attaching to devices (blocks)

Disk Scheduling
 The operating system is responsible for using hardware
efficiently — for the disk drives, this means having a fast
access time and disk bandwidth
 Minimize seek time
 Seek time  seek distance
 Disk bandwidth is the total number of bytes transferred,
divided by the total time between the first request for service
and the completion of the last transfer

Disk Scheduling (Cont.)
 There are many sources of disk I/O request
 OS
 System processes
 Users processes
 I/O request includes input or output mode, disk address, memory
address, number of sectors to transfer
 OS maintains queue of requests, per disk or device
 Idle disk can immediately work on I/O request, busy disk means
work must queue
 Optimization algorithms only make sense when a queue exists

Disk Scheduling (Cont.)
 Note that drive controllers have small buffers and can manage a
queue of I/O requests (of varying “depth”)
 Several algorithms exist to schedule the servicing of disk I/O
requests
 The analysis is true for one or many platters
 We illustrate scheduling algorithms with a request queue (0-199)
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53

FCFS
Illustration shows total head movement of 640 cylinders

SSTF
 Shortest Seek Time First selects the request with the minimum
seek time from the current head position
 SSTF scheduling is a form of SJF scheduling; may cause
starvation of some requests
 Illustration shows total head movement of 236 cylinders

SCAN
 The disk arm starts at one end of the disk, and moves toward the
other end, servicing requests until it gets to the other end of the
disk, where the head movement is reversed and servicing
continues.
 SCAN algorithm Sometimes called the elevator algorithm
 Illustration shows total head movement of 236 cylinders
 But note that if requests are uniformly dense, largest density at
other end of disk and those wait the longest

SCAN (Cont.)

C-SCAN
 Provides a more uniform wait time than SCAN
 The head moves from one end of the disk to the other, servicing
requests as it goes
 When it reaches the other end, however, it immediately
returns to the beginning of the disk, without servicing any
requests on the return trip
 Treats the cylinders as a circular list that wraps around from the
last cylinder to the first one
 Total number of cylinders?

C-SCAN (Cont.)

C-LOOK
 LOOK a version of SCAN, C-LOOK a version of C-SCAN
 Arm only goes as far as the last request in each direction,
then reverses direction immediately, without first going all
the way to the end of the disk
 Total number of cylinders?

C-LOOK (Cont.)

Selecting a Disk-Scheduling Algorithm
 SSTF is common and has a natural appeal
 SCAN and C-SCAN perform better for systems that place a heavy load
on the disk
 Less starvation
 Performance depends on the number and types of requests
 Requests for disk service can be influenced by the file-allocation method
 And metadata layout
 The disk-scheduling algorithm should be written as a separate module of
the operating system, allowing it to be replaced with a different algorithm
if necessary
 Either SSTF or LOOK is a reasonable choice for the default algorithm
 What about rotational latency?
 Difficult for OS to calculate
 How does disk-based queueing effect OS queue ordering efforts?

Disk Management
 Low-level formatting, or physical formatting — Dividing a disk into
sectors that the disk controller can read and write
 Each sector can hold header information, plus data, plus error
correction code (ECC)
 Usually 512 bytes of data but can be selectable
 To use a disk to hold files, the operating system still needs to record its
own data structures on the disk
 Partition the disk into one or more groups of cylinders, each treated
as a logical disk
 Logical formatting or “making a file system”
 To increase efficiency most file systems group blocks into clusters
 Disk I/O done in blocks
 File I/O done in clusters

Disk Management (Cont.)
 Raw disk access for apps that want to do their own block
management, keep OS out of the way (databases for example)
 Boot block initializes system
 The bootstrap is stored in ROM
 Bootstrap loader program stored in boot blocks of boot
partition
 Methods such as sector sparing used to handle bad blocks

Booting from a Disk in Windows

Swap-Space Management
 Swap-space — Virtual memory uses disk space as an extension of main memory
 Less common now due to memory capacity increases
 Swap-space can be carved out of the normal file system, or, more commonly, it
can be in a separate disk partition (raw)
 Swap-space management
 4.3BSD allocates swap space when process starts; holds text segment (the
program) and data segment
 Kernel uses swap maps to track swap-space use
 Solaris 2 allocates swap space only when a dirty page is forced out of
physical memory, not when the virtual memory page is first created
 File data written to swap space until write to file system requested
 Other dirty pages go to swap space due to no other home
 Text segment pages thrown out and reread from the file system as
needed
 What if a system runs out of swap space?
 Some systems allow multiple swap spaces

Data Structures for Swapping on Linux Systems

RAID Structure
 RAID – redundant array of inexpensive disks
 multiple disk drives provides reliability via redundancy
 Increases the mean time to failure
 Mean time to repair – exposure time when another failure could
cause data loss
 Mean time to data loss based on above factors
 If mirrored disks fail independently, consider disk with 1300,000
mean time to failure and 10 hour mean time to repair
 Mean time to data loss is 100, 0002 / (2 ∗ 10) = 500 ∗ 106 hours,
or 57,000 years!
 Frequently combined with NVRAM to improve write performance
 Several improvements in disk-use techniques involve the use of
multiple disks working cooperatively

RAID (Cont.)
 Disk striping uses a group of disks as one storage unit
 RAID is arranged into six different levels
 RAID schemes improve performance and improve the reliability
of the storage system by storing redundant data
 Mirroring or shadowing (RAID 1) keeps duplicate of each
disk
 Striped mirrors (RAID 1+0) or mirrored stripes (RAID 0+1)
provides high performance and high reliability
 Block interleaved parity (RAID 4, 5, 6) uses much less
redundancy
 RAID within a storage array can still fail if the array fails, so
automatic replication of the data between arrays is common
 Frequently, a small number of hot-spare disks are left
unallocated, automatically replacing a failed disk and having data
rebuilt onto them

RAID Levels

RAID (0 + 1) and (1 + 0)

Other Features
 Regardless of where RAID implemented, other useful features
can be added
 Snapshot is a view of file system before a set of changes take
place (i.e. at a point in time)
 More in Ch 12
 Replication is automatic duplication of writes between separate
sites
 For redundancy and disaster recovery
 Can be synchronous or asynchronous
 Hot spare disk is unused, automatically used by RAID production
if a disk fails to replace the failed disk and rebuild the RAID set if
possible
 Decreases mean time to repair

Extensions
 RAID alone does not prevent or detect data corruption or other
errors, just disk failures
 Solaris ZFS adds checksums of all data and metadata
 Checksums kept with pointer to object, to detect if object is the
right one and whether it changed
 Can detect and correct data and metadata corruption
 ZFS also removes volumes, partitions
 Disks allocated in pools
 Filesystems with a pool share that pool, use and release
space like malloc() and free() memory allocate /
release calls

ZFS Checksums All Metadata and Data

Traditional and Pooled Storage

Stable-Storage Implementation
 Write-ahead log scheme requires stable storage
 Stable storage means data is never lost (due to failure, etc)
 To implement stable storage:
 Replicate information on more than one nonvolatile storage media
with independent failure modes
 Update information in a controlled manner to ensure that we can
recover the stable data after any failure during data transfer or
recovery
 Disk write has 1 of 3 outcomes
1. Successful completion - The data were written correctly on disk
2. Partial failure - A failure occurred in the midst of transfer, so only
some of the sectors were written with the new data, and the sector
being written during the failure may have been corrupted
3. Total failure - The failure occurred before the disk write started, so
the previous data values on the disk remain intact

Stable-Storage Implementation (Cont.)
 If failure occurs during block write, recovery procedure restores
block to consistent state
 System maintains 2 physical blocks per logical block and
does the following:
1. Write to 1st physical
2. When successful, write to 2nd physical
3. Declare complete only after second write completes
successfully
Systems frequently use NVRAM as one physical to accelerate

End of Chapter 10

Chapter 11:
File-System Interface

Chapter 11: File-System Interface
 File Concept
 Access Methods
 Disk and Directory Structure
 File-System Mounting
 File Sharing
 Protection

Objectives
 To explain the function of file systems
 To describe the interfaces to file systems
 To discuss file-system design tradeoffs, including access
methods, file sharing, file locking, and directory structures
 To explore file-system protection

File Concept
 Contiguous logical address space
 Types:
 Data
 numeric
 character
 binary
 Program
 Contents defined by file’s creator
 Many types
 Consider text file, source file, executable file

File Attributes
 Name – only information kept in human-readable form
 Identifier – unique tag (number) identifies file within file system
 Type – needed for systems that support different types
 Location – pointer to file location on device
 Size – current file size
 Protection – controls who can do reading, writing, executing
 Time, date, and user identification – data for protection, security,
and usage monitoring
 Information about files are kept in the directory structure, which is
maintained on the disk
 Many variations, including extended file attributes such as file
checksum
 Information kept in the directory structure

File info Window on Mac OS X

File Operations
 File is an abstract data type
 Create
 Write – at write pointer location
 Read – at read pointer location
 Reposition within file - seek
 Delete
 Truncate
 Open(Fi) – search the directory structure on disk for entry Fi,
and move the content of entry to memory
 Close (Fi) – move the content of entry Fi in memory to
directory structure on disk

Open Files
 Several pieces of data are needed to manage open files:
 Open-file table: tracks open files
 File pointer: pointer to last read/write location, per
process that has the file open
 File-open count: counter of number of times a file is
open – to allow removal of data from open-file table when
last processes closes it
 Disk location of the file: cache of data access information
 Access rights: per-process access mode information

Open File Locking
 Provided by some operating systems and file systems
 Similar to reader-writer locks
 Shared lock similar to reader lock – several processes can
acquire concurrently
 Exclusive lock similar to writer lock
 Mediates access to a file
 Mandatory or advisory:
 Mandatory – access is denied depending on locks held and
requested
 Advisory – processes can find status of locks and decide
what to do

File Locking Example – Java API
import java.io.*;
import java.nio.channels.*;
public class LockingExample {
public static final boolean EXCLUSIVE = false;
public static final boolean SHARED = true;
public static void main(String arsg[]) throws IOException {
FileLock sharedLock = null;
FileLock exclusiveLock = null;
try {
RandomAccessFile raf = new RandomAccessFile("file.txt", "rw");
// get the channel for the file
FileChannel ch = raf.getChannel();
// this locks the first half of the file - exclusive
exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);
/** Now modify the data . . . */
// release the lock
exclusiveLock.release();

File Locking Example – Java API (Cont.)
// this locks the second half of the file - shared
sharedLock = ch.lock(raf.length()/2+1, raf.length(),
SHARED);
/** Now read the data . . . */
// release the lock
sharedLock.release();
} catch (java.io.IOException ioe) {
System.err.println(ioe);
}finally {
if (exclusiveLock != null)
exclusiveLock.release();
if (sharedLock != null)
sharedLock.release();
}
}
}

File Types – Name, Extension

File Structure
 None - sequence of words, bytes
 Simple record structure
 Lines
 Fixed length
 Variable length
 Complex Structures
 Formatted document
 Relocatable load file
 Can simulate last two with first method by inserting
appropriate control characters
 Who decides:
 Operating system
 Program

Sequential-access File

Access Methods
 Sequential Access
read next
write next
reset
no read after last write
(rewrite)
 Direct Access – file is fixed length logical records
read n
write n
position to n
read next
write next
rewrite n
n = relative block number
 Relative block numbers allow OS to decide where file should be placed
 See allocation problem in Ch 12

Simulation of Sequential Access on Direct-access File

Other Access Methods
 Can be built on top of base methods
 General involve creation of an index for the file
 Keep index in memory for fast determination of location of
data to be operated on (consider UPC code plus record of
data about that item)
 If too large, index (in memory) of the index (on disk)
 IBM indexed sequential-access method (ISAM)
 Small master index, points to disk blocks of secondary
index
 File kept sorted on a defined key
 All done by the OS
 VMS operating system provides index and relative files as
another example (see next slide)

Example of Index and Relative Files

Directory Structure
 A collection of nodes containing information about all files
F 1 F 2
F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on disk

Disk Structure
 Disk can be subdivided into partitions
 Disks or partitions can be RAID protected against failure
 Disk or partition can be used raw – without a file system, or
formatted with a file system
 Partitions also known as minidisks, slices
 Entity containing file system known as a volume
 Each volume containing file system also tracks that file
system’s info in device directory or volume table of contents
 As well as general-purpose file systems there are many
special-purpose file systems, frequently all within the same
operating system or computer

A Typical File-system Organization

Types of File Systems
 We mostly talk of general-purpose file systems
 But systems frequently have may file systems, some general- and
some special- purpose
 Consider Solaris has
 tmpfs – memory-based volatile FS for fast, temporary I/O
 objfs – interface into kernel memory to get kernel symbols for
debugging
 ctfs – contract file system for managing daemons
 lofs – loopback file system allows one FS to be accessed in
place of another
 procfs – kernel interface to process structures
 ufs, zfs – general purpose file systems

Operations Performed on Directory
 Search for a file
 Create a file
 Delete a file
 List a directory
 Rename a file
 Traverse the file system

Directory Organization
 Efficiency – locating a file quickly
 Naming – convenient to users
 Two users can have same name for different files
 The same file can have several different names
 Grouping – logical grouping of files by properties, (e.g., all
Java programs, all games, …)
The directory is organized logically to obtain

Single-Level Directory
 A single directory for all users
 Naming problem
 Grouping problem

Two-Level Directory
 Separate directory for each user
 Path name
 Can have the same file name for different user
 Efficient searching
 No grouping capability

Tree-Structured Directories

Tree-Structured Directories (Cont.)
 Efficient searching
 Grouping Capability
 Current directory (working directory)
 cd /spell/mail/prog
 type list

Tree-Structured Directories (Cont)
 Absolute or relative path name
 Creating a new file is done in current directory
 Delete a file
rm <file-name>
 Creating a new subdirectory is done in current directory
mkdir <dir-name>
Example: if in current directory /mail
mkdir count
Deleting “mail”  deleting the entire subtree rooted by “mail”

Acyclic-Graph Directories
 Have shared subdirectories and files

Acyclic-Graph Directories (Cont.)
 Two different names (aliasing)
 If dict deletes list  dangling pointer
Solutions:
 Backpointers, so we can delete all pointers
Variable size records a problem
 Backpointers using a daisy chain organization
 Entry-hold-count solution
 New directory entry type
 Link – another name (pointer) to an existing file
 Resolve the link – follow pointer to locate the file

General Graph Directory

General Graph Directory (Cont.)
 How do we guarantee no cycles?
 Allow only links to file not subdirectories
 Garbage collection
 Every time a new link is added use a cycle detection
algorithm to determine whether it is OK

File System Mounting
 A file system must be mounted before it can be accessed
 A unmounted file system (i.e., Fig. 11-11(b)) is mounted at a
mount point

Mount Point

File Sharing
 Sharing of files on multi-user systems is desirable
 Sharing may be done through a protection scheme
 On distributed systems, files may be shared across a network
 Network File System (NFS) is a common distributed file-sharing
method
 If multi-user system
 User IDs identify users, allowing permissions and
protections to be per-user
Group IDs allow users to be in groups, permitting group
access rights
 Owner of a file / directory
 Group of a file / directory

File Sharing – Remote File Systems
 Uses networking to allow file system access between systems
 Manually via programs like FTP
 Automatically, seamlessly using distributed file systems
 Semi automatically via the world wide web
 Client-server model allows clients to mount remote file systems from
servers
 Server can serve multiple clients
 Client and user-on-client identification is insecure or complicated
 NFS is standard UNIX client-server file sharing protocol
 CIFS is standard Windows protocol
 Standard operating system file calls are translated into remote calls
 Distributed Information Systems (distributed naming services) such
as LDAP, DNS, NIS, Active Directory implement unified access to
information needed for remote computing

File Sharing – Failure Modes
 All file systems have failure modes
 For example corruption of directory structures or other non-
user data, called metadata
 Remote file systems add new failure modes, due to network
failure, server failure
 Recovery from failure can involve state information about
status of each remote request
 Stateless protocols such as NFS v3 include all information in
each request, allowing easy recovery but less security

File Sharing – Consistency Semantics
 Specify how multiple users are to access a shared file
simultaneously
 Similar to Ch 5 process synchronization algorithms
 Tend to be less complex due to disk I/O and network
latency (for remote file systems
 Andrew File System (AFS) implemented complex remote file
sharing semantics
 Unix file system (UFS) implements:
 Writes to an open file visible immediately to other users of
the same open file
 Sharing file pointer to allow multiple users to read and write
concurrently
 AFS has session semantics
 Writes only visible to sessions starting after the file is
closed

Protection
 File owner/creator should be able to control:
 what can be done
 by whom
 Types of access
 Read
 Write
 Execute
 Append
 Delete
 List

Access Lists and Groups
 Mode of access: read, write, execute
 Three classes of users on Unix / Linux
RWX
a) owner access 7  1 1 1
RWX
b) group access 6  1 1 0
RWX
c) public access 1  0 0 1
 Ask manager to create a group (unique name), say G, and add
some users to the group.
 For a particular file (say game) or subdirectory, define an
appropriate access.
Attach a group to a file
chgrp G game

Windows 7 Access-Control List Management

A Sample UNIX Directory Listing

End of Chapter 11

Chapter 12: File System
Implementation

Chapter 12: File System Implementation
 File-System Structure
 File-System Implementation
 Directory Implementation
 Allocation Methods
 Free-Space Management
 Efficiency and Performance
 Recovery
 NFS
 Example: WAFL File System

Objectives
 To describe the details of implementing local file systems and
directory structures
 To describe the implementation of remote file systems
 To discuss block allocation and free-block algorithms and trade-
offs

File-System Structure
 File structure
 Logical storage unit
 Collection of related information
 File system resides on secondary storage (disks)
 Provided user interface to storage, mapping logical to physical
 Provides efficient and convenient access to disk by allowing
data to be stored, located retrieved easily
 Disk provides in-place rewrite and random access
 I/O transfers performed in blocks of sectors (usually 512
bytes)
 File control block – storage structure consisting of information
about a file
 Device driver controls the physical device
 File system organized into layers

Layered File System

File System Layers
 Device drivers manage I/O devices at the I/O control layer
 Given commands like “read drive1, cylinder 72, track 2, sector
10, into memory location 1060” outputs low-level hardware
specific commands to hardware controller
 Basic file system given command like “retrieve block 123”
translates to device driver
 Also manages memory buffers and caches (allocation, freeing,
replacement)
 Buffers hold data in transit
 Caches hold frequently used data
 File organization module understands files, logical address, and
physical blocks
 Translates logical block # to physical block #
 Manages free space, disk allocation

File System Layers (Cont.)
 Logical file system manages metadata information
 Translates file name into file number, file handle, location by
maintaining file control blocks (inodes in UNIX)
 Directory management
 Protection
 Layering useful for reducing complexity and redundancy, but
adds overhead and can decrease performanceTranslates file
name into file number, file handle, location by maintaining file
control blocks (inodes in UNIX)
 Logical layers can be implemented by any coding method
according to OS designer

File System Layers (Cont.)
 Many file systems, sometimes many within an operating
system
 Each with its own format (CD-ROM is ISO 9660; Unix has
UFS, FFS; Windows has FAT, FAT32, NTFS as well as
floppy, CD, DVD Blu-ray, Linux has more than 40 types,
with extended file system ext2 and ext3 leading; plus
distributed file systems, etc.)
 New ones still arriving – ZFS, GoogleFS, Oracle ASM,
FUSE

File-System Implementation
 We have system calls at the API level, but how do we implement
their functions?
 On-disk and in-memory structures
 Boot control block contains info needed by system to boot OS
from that volume
 Needed if volume contains OS, usually first block of volume
 Volume control block (superblock, master file table) contains
volume details
 Total # of blocks, # of free blocks, block size, free block
pointers or array
 Directory structure organizes the files
 Names and inode numbers, master file table

File-System Implementation (Cont.)
 Per-file File Control Block (FCB) contains many details about
the file
 inode number, permissions, size, dates
 NFTS stores into in master file table using relational DB
structures

In-Memory File System Structures
 Mount table storing file system mounts, mount points, file
system types
 The following figure illustrates the necessary file system
structures provided by the operating systems
 Figure 12-3(a) refers to opening a file
 Figure 12-3(b) refers to reading a file
 Plus buffers hold data blocks from secondary storage
 Open returns a file handle for subsequent use
 Data from read eventually copied to specified user process
memory address

In-Memory File System Structures

Partitions and Mounting
 Partition can be a volume containing a file system (“cooked”) or
raw – just a sequence of blocks with no file system
 Boot block can point to boot volume or boot loader set of blocks that
contain enough code to know how to load the kernel from the file
system
 Or a boot management program for multi-os booting
 Root partition contains the OS, other partitions can hold other
Oses, other file systems, or be raw
 Mounted at boot time
 Other partitions can mount automatically or manually
 At mount time, file system consistency checked
 Is all metadata correct?
 If not, fix it, try again
 If yes, add to mount table, allow access

Virtual File Systems
 Virtual File Systems (VFS) on Unix provide an object-oriented
way of implementing file systems
 VFS allows the same system call interface (the API) to be used
for different types of file systems
 Separates file-system generic operations from
implementation details
 Implementation can be one of many file systems types, or
network file system
 Implements vnodes which hold inodes or network file
details
 Then dispatches operation to appropriate file system
implementation routines

Virtual File Systems (Cont.)
 The API is to the VFS interface, rather than any specific type of
file system

Virtual File System Implementation
 For example, Linux has four object types:
 inode, file, superblock, dentry
 VFS defines set of operations on the objects that must be
implemented
 Every object has a pointer to a function table
 Function table has addresses of routines to implement that
function on that object
 For example:
 • int open(. . .)—Open a file
 • int close(. . .)—Close an already-open file
 • ssize t read(. . .)—Read from a file
 • ssize t write(. . .)—Write to a file
 • int mmap(. . .)—Memory-map a file

Directory Implementation
 Linear list of file names with pointer to the data blocks
 Simple to program
 Time-consuming to execute
 Linear search time
 Could keep ordered alphabetically via linked list or use
B+ tree
 Hash Table – linear list with hash data structure
 Decreases directory search time
 Collisions – situations where two file names hash to the
same location
 Only good if entries are fixed size, or use chained-overflow
method

Allocation Methods - Contiguous
 An allocation method refers to how disk blocks are allocated for
files:
 Contiguous allocation – each file occupies set of contiguous
blocks
 Best performance in most cases
 Simple – only starting location (block #) and length (number
of blocks) are required
 Problems include finding space for file, knowing file size,
external fragmentation, need for compaction off-line
(downtime) or on-line

Contiguous Allocation
 Mapping from logical to physical
LA/512
Q
R
Block to be accessed = Q +
starting address
Displacement into block = R

Extent-Based Systems
 Many newer file systems (i.e., Veritas File System) use a
modified contiguous allocation scheme
 Extent-based file systems allocate disk blocks in extents
 An extent is a contiguous block of disks
 Extents are allocated for file allocation
 A file consists of one or more extents

Allocation Methods - Linked
 Linked allocation – each file a linked list of blocks
 File ends at nil pointer
 No external fragmentation
 Each block contains pointer to next block
 No compaction, external fragmentation
 Free space management system called when new block
needed
 Improve efficiency by clustering blocks into groups but
increases internal fragmentation
 Reliability can be a problem
 Locating a block can take many I/Os and disk seeks

Allocation Methods – Linked (Cont.)
 FAT (File Allocation Table) variation
 Beginning of volume has table, indexed by block number
 Much like a linked list, but faster on disk and cacheable
 New block allocation simple

Linked Allocation
 Each file is a linked list of disk blocks: blocks may be scattered
anywhere on the disk
pointer
block =
 Mapping
Block to be accessed is the Qth block in the linked chain of blocks
representing the file.
Displacement into block = R + 1
LA/511
Q
R

Linked Allocation

File-Allocation Table

Allocation Methods - Indexed
 Indexed allocation
 Each file has its own index block(s) of pointers to its data blocks
 Logical view
index table

Example of Indexed Allocation

Indexed Allocation (Cont.)
 Need index table
 Random access
 Dynamic access without external fragmentation, but have overhead
of index block
 Mapping from logical to physical in a file of maximum size of 256K
bytes and block size of 512 bytes. We need only 1 block for index
table
LA/512
Q
R
Q = displacement into index table
R = displacement into block

Indexed Allocation – Mapping (Cont.)
 Mapping from logical to physical in a file of unbounded length (block
size of 512 words)
 Linked scheme – Link blocks of index table (no limit on size)
LA / (512 x 511)
Q1
R1
Q1 = block of index table
R1 is used as follows:
R1 / 512
Q2
R2
Q2 = displacement into block of index table
R2 displacement into block of file:

 Two-level index (4K blocks could store 1,024 four-byte pointers in outer
index -> 1,048,567 data blocks and file size of up to 4GB)
LA / (512 x 512)
Q1
R1
Q1 = displacement into outer-index
R1 is used as follows:
R1 / 512
Q2
R2
Q2 = displacement into block of index table
R2 displacement into block of file:

Combined Scheme: UNIX UFS
More index blocks than can be addressed with 32-bit file pointer
4K bytes per block, 32-bit addresses

Performance
 Best method depends on file access type
 Contiguous great for sequential and random
 Linked good for sequential, not random
 Declare access type at creation -> select either contiguous or
linked
 Indexed more complex
 Single block access could require 2 index block reads then
data block read
 Clustering can help improve throughput, reduce CPU
overhead

Performance (Cont.)
 Adding instructions to the execution path to save one disk I/O is
reasonable
 Intel Core i7 Extreme Edition 990x (2011) at 3.46Ghz = 159,000
MIPS
 http://en.wikipedia.org/wiki/Instructions_per_second
 Typical disk drive at 250 I/Os per second
 159,000 MIPS / 250 = 630 million instructions during one
disk I/O
 Fast SSD drives provide 60,000 IOPS
 159,000 MIPS / 60,000 = 2.65 millions instructions during
one disk I/O

Free-Space Management
 File system maintains free-space list to track available blocks/clusters
 (Using term “block” for simplicity)
 Bit vector or bit map (n blocks)
…
0 1 2 n-1
bit[i] =

1  block[i] free
0  block[i] occupied
Block number calculation
(number of bits per word) *
(number of 0-value words) +
offset of first 1 bit
CPUs have instructions to return offset within word of first “1” bit

Free-Space Management (Cont.)
 Bit map requires extra space
 Example:
block size = 4KB = 212 bytes
disk size = 240 bytes (1 terabyte)
n = 240/212 = 228 bits (or 32MB)
if clusters of 4 blocks -> 8MB of memory
 Easy to get contiguous files

Linked Free Space List on Disk
 Linked list (free list)
 Cannot get contiguous
space easily
 No waste of space
 No need to traverse the
entire list (if # free blocks
recorded)

 Grouping
 Modify linked list to store address of next n-1 free blocks in first
free block, plus a pointer to next block that contains free-block-
pointers (like this one)
 Counting
 Because space is frequently contiguously used and freed, with
contiguous-allocation allocation, extents, or clustering
 Keep address of first free block and count of following free
blocks
 Free space list then has entries containing addresses and
counts

 Space Maps
 Used in ZFS
 Consider meta-data I/O on very large file systems
 Full data structures like bit maps couldn’t fit in memory ->
thousands of I/Os
 Divides device space into metaslab units and manages metaslabs
 Given volume can contain hundreds of metaslabs
 Each metaslab has associated space map
 Uses counting algorithm
 But records to log file rather than file system
 Log of all block activity, in time order, in counting format
 Metaslab activity -> load space map into memory in balanced-tree
structure, indexed by offset
 Replay log into that structure
 Combine contiguous free blocks into single entry

Efficiency and Performance
 Efficiency dependent on:
 Disk allocation and directory algorithms
 Types of data kept in file’s directory entry
 Pre-allocation or as-needed allocation of metadata
structures
 Fixed-size or varying-size data structures

Efficiency and Performance (Cont.)
 Performance
 Keeping data and metadata close together
 Buffer cache – separate section of main memory for frequently
used blocks
 Synchronous writes sometimes requested by apps or needed
by OS
 No buffering / caching – writes must hit disk before
acknowledgement
 Asynchronous writes more common, buffer-able, faster
 Free-behind and read-ahead – techniques to optimize
sequential access
 Reads frequently slower than writes

Page Cache
 A page cache caches pages rather than disk blocks using virtual
memory techniques and addresses
 Memory-mapped I/O uses a page cache
 Routine I/O through the file system uses the buffer (disk) cache
 This leads to the following figure

I/O Without a Unified Buffer Cache

Unified Buffer Cache
 A unified buffer cache uses the same page cache to cache
both memory-mapped pages and ordinary file system I/O to
avoid double caching
 But which caches get priority, and what replacement
algorithms to use?

I/O Using a Unified Buffer Cache

Recovery
 Consistency checking – compares data in directory structure
with data blocks on disk, and tries to fix inconsistencies
 Can be slow and sometimes fails
 Use system programs to back up data from disk to another
storage device (magnetic tape, other magnetic disk, optical)
 Recover lost file or disk by restoring data from backup

Log Structured File Systems
 Log structured (or journaling) file systems record each metadata
update to the file system as a transaction
 All transactions are written to a log
 A transaction is considered committed once it is written to the
log (sequentially)
 Sometimes to a separate device or section of disk
 However, the file system may not yet be updated
 The transactions in the log are asynchronously written to the file
system structures
 When the file system structures are modified, the transaction is
removed from the log
 If the file system crashes, all remaining transactions in the log must
still be performed
 Faster recovery from crash, removes chance of inconsistency of
metadata

The Sun Network File System (NFS)
 An implementation and a specification of a software system
for accessing remote files across LANs (or WANs)
 The implementation is part of the Solaris and SunOS
operating systems running on Sun workstations using an
unreliable datagram protocol (UDP/IP protocol and Ethernet

NFS (Cont.)
 Interconnected workstations viewed as a set of independent machines
with independent file systems, which allows sharing among these file
systems in a transparent manner
 A remote directory is mounted over a local file system directory
 The mounted directory looks like an integral subtree of the local
file system, replacing the subtree descending from the local
directory
 Specification of the remote directory for the mount operation is
nontransparent; the host name of the remote directory has to be
provided
 Files in the remote directory can then be accessed in a
transparent manner
 Subject to access-rights accreditation, potentially any file system
(or directory within a file system), can be mounted remotely on top
of any local directory

NFS (Cont.)
 NFS is designed to operate in a heterogeneous environment of
different machines, operating systems, and network architectures;
the NFS specifications independent of these media
 This independence is achieved through the use of RPC primitives
built on top of an External Data Representation (XDR) protocol
used between two implementation-independent interfaces
 The NFS specification distinguishes between the services provided
by a mount mechanism and the actual remote-file-access services

Three Independent File Systems

Mounting in NFS
Mounts Cascading mounts

NFS Mount Protocol
 Establishes initial logical connection between server and client
 Mount operation includes name of remote directory to be mounted
and name of server machine storing it
 Mount request is mapped to corresponding RPC and forwarded
to mount server running on server machine
 Export list – specifies local file systems that server exports for
mounting, along with names of machines that are permitted to
mount them
 Following a mount request that conforms to its export list, the
server returns a file handle—a key for further accesses
 File handle – a file-system identifier, and an inode number to
identify the mounted directory within the exported file system
 The mount operation changes only the user’s view and does not
affect the server side

NFS Protocol
 Provides a set of remote procedure calls for remote file operations.
The procedures support the following operations:
 searching for a file within a directory
 reading a set of directory entries
 manipulating links and directories
 accessing file attributes
 reading and writing files
 NFS servers are stateless; each request has to provide a full set
of arguments (NFS V4 is just coming available – very different,
stateful)
 Modified data must be committed to the server’s disk before
results are returned to the client (lose advantages of caching)
 The NFS protocol does not provide concurrency-control
mechanisms

Three Major Layers of NFS Architecture
 UNIX file-system interface (based on the open, read, write, and
close calls, and file descriptors)
 Virtual File System (VFS) layer – distinguishes local files from
remote ones, and local files are further distinguished according to
their file-system types
 The VFS activates file-system-specific operations to handle
local requests according to their file-system types
 Calls the NFS protocol procedures for remote requests
 NFS service layer – bottom layer of the architecture
 Implements the NFS protocol

Schematic View of NFS Architecture

NFS Path-Name Translation
 Performed by breaking the path into component names and
performing a separate NFS lookup call for every pair of
component name and directory vnode
 To make lookup faster, a directory name lookup cache on the
client’s side holds the vnodes for remote directory names

NFS Remote Operations
 Nearly one-to-one correspondence between regular UNIX system
calls and the NFS protocol RPCs (except opening and closing
files)
 NFS adheres to the remote-service paradigm, but employs
buffering and caching techniques for the sake of performance
 File-blocks cache – when a file is opened, the kernel checks with
the remote server whether to fetch or revalidate the cached
attributes
 Cached file blocks are used only if the corresponding cached
attributes are up to date
 File-attribute cache – the attribute cache is updated whenever new
attributes arrive from the server
 Clients do not free delayed-write blocks until the server confirms
that the data have been written to disk

Example: WAFL File System
 Used on Network Appliance “Filers” – distributed file system
appliances
 “Write-anywhere file layout”
 Serves up NFS, CIFS, http, ftp
 Random I/O optimized, write optimized
 NVRAM for write caching
 Similar to Berkeley Fast File System, with extensive
modifications

The WAFL File Layout

Snapshots in WAFL

End of Chapter 12

Chapter 13: I/O Systems

Chapter 13: I/O Systems
 Overview
 I/O Hardware
 Application I/O Interface
 Kernel I/O Subsystem
 Transforming I/O Requests to Hardware Operations
 STREAMS
 Performance

Objectives
 Explore the structure of an operating system’s I/O subsystem
 Discuss the principles of I/O hardware and its complexity
 Provide details of the performance aspects of I/O hardware
and software

Overview
 I/O management is a major component of operating system
design and operation
 Important aspect of computer operation
 I/O devices vary greatly
 Various methods to control them
 Performance management
 New types of devices frequent
 Ports, busses, device controllers connect to various devices
 Device drivers encapsulate device details
 Present uniform device-access interface to I/O subsystem

I/O Hardware
 Incredible variety of I/O devices
 Storage
 Transmission
 Human-interface
 Common concepts – signals from I/O devices interface with computer
 Port – connection point for device
 Bus - daisy chain or shared direct access
 PCI bus common in PCs and servers, PCI Express (PCIe)
 expansion bus connects relatively slow devices
 Controller (host adapter) – electronics that operate port, bus, device
 Sometimes integrated
 Sometimes separate circuit board (host adapter)
 Contains processor, microcode, private memory, bus controller, etc
– Some talk to per-device controller with bus controller, microcode,
memory, etc

A Typical PC Bus Structure

I/O Hardware (Cont.)
 I/O instructions control devices
 Devices usually have registers where device driver places
commands, addresses, and data to write, or read data from
registers after command execution
 Data-in register, data-out register, status register, control
register
 Typically 1-4 bytes, or FIFO buffer
 Devices have addresses, used by
 Direct I/O instructions
 Memory-mapped I/O
 Device data and command registers mapped to
processor address space
 Especially for large address spaces (graphics)

Device I/O Port Locations on PCs (partial)

Polling
 For each byte of I/O
1. Read busy bit from status register until 0
2. Host sets read or write bit and if write copies data into data-out
register
3. Host sets command-ready bit
4. Controller sets busy bit, executes transfer
5. Controller clears busy bit, error bit, command-ready bit when
transfer done
 Step 1 is busy-wait cycle to wait for I/O from device
 Reasonable if device is fast
 But inefficient if device slow
 CPU switches to other tasks?
 But if miss a cycle data overwritten / lost

Interrupts
 Polling can happen in 3 instruction cycles
 Read status, logical-and to extract status bit, branch if not zero
 How to be more efficient if non-zero infrequently?
 CPU Interrupt-request line triggered by I/O device
 Checked by processor after each instruction
 Interrupt handler receives interrupts
 Maskable to ignore or delay some interrupts
 Interrupt vector to dispatch interrupt to correct handler
 Context switch at start and end
 Based on priority
 Some nonmaskable
 Interrupt chaining if more than one device at same interrupt
number

Interrupt-Driven I/O Cycle

Intel Pentium Processor Event-Vector Table

Interrupts (Cont.)
 Interrupt mechanism also used for exceptions
 Terminate process, crash system due to hardware error
 Page fault executes when memory access error
 System call executes via trap to trigger kernel to execute
request
 Multi-CPU systems can process interrupts concurrently
 If operating system designed to handle it
 Used for time-sensitive processing, frequent, must be fast

Direct Memory Access
 Used to avoid programmed I/O (one byte at a time) for large data
movement
 Requires DMA controller
 Bypasses CPU to transfer data directly between I/O device and
memory
 OS writes DMA command block into memory
 Source and destination addresses
 Read or write mode
 Count of bytes
 Writes location of command block to DMA controller
 Bus mastering of DMA controller – grabs bus from CPU
 Cycle stealing from CPU but still much more efficient
 When done, interrupts to signal completion
 Version that is aware of virtual addresses can be even more efficient -
DVMA

Six Step Process to Perform DMA Transfer

Application I/O Interface
 I/O system calls encapsulate device behaviors in generic classes
 Device-driver layer hides differences among I/O controllers from kernel
 New devices talking already-implemented protocols need no extra
work
 Each OS has its own I/O subsystem structures and device driver
frameworks
 Devices vary in many dimensions
 Character-stream or block
 Sequential or random-access
 Synchronous or asynchronous (or both)
 Sharable or dedicated
 Speed of operation
 read-write, read only, or write only

A Kernel I/O Structure

Characteristics of I/O Devices

Characteristics of I/O Devices (Cont.)
 Subtleties of devices handled by device drivers
 Broadly I/O devices can be grouped by the OS into
 Block I/O
 Character I/O (Stream)
 Memory-mapped file access
 Network sockets
 For direct manipulation of I/O device specific characteristics,
usually an escape / back door
 Unix ioctl() call to send arbitrary bits to a device control
register and data to device data register

Block and Character Devices
 Block devices include disk drives
 Commands include read, write, seek
 Raw I/O, direct I/O, or file-system access
 Memory-mapped file access possible
 File mapped to virtual memory and clusters brought via
demand paging
 DMA
 Character devices include keyboards, mice, serial ports
 Commands include get(), put()
 Libraries layered on top allow line editing

Network Devices
 Varying enough from block and character to have own
interface
 Linux, Unix, Windows and many others include socket
interface
 Separates network protocol from network operation
 Includes select() functionality
 Approaches vary widely (pipes, FIFOs, streams, queues,
mailboxes)

Clocks and Timers
 Provide current time, elapsed time, timer
 Normal resolution about 1/60 second
 Some systems provide higher-resolution timers
 Programmable interval timer used for timings, periodic
interrupts
 ioctl() (on UNIX) covers odd aspects of I/O such as
clocks and timers

Nonblocking and Asynchronous I/O
 Blocking - process suspended until I/O completed
 Easy to use and understand
 Insufficient for some needs
 Nonblocking - I/O call returns as much as available
 User interface, data copy (buffered I/O)
 Implemented via multi-threading
 Returns quickly with count of bytes read or written
 select() to find if data ready then read() or write()
to transfer
 Asynchronous - process runs while I/O executes
 Difficult to use
 I/O subsystem signals process when I/O completed

Two I/O Methods
Synchronous Asynchronous

Vectored I/O
 Vectored I/O allows one system call to perform multiple I/O
operations
 For example, Unix readve() accepts a vector of multiple
buffers to read into or write from
 This scatter-gather method better than multiple individual I/O
calls
 Decreases context switching and system call overhead
 Some versions provide atomicity
 Avoid for example worry about multiple threads
changing data as reads / writes occurring

Kernel I/O Subsystem
 Scheduling
 Some I/O request ordering via per-device queue
 Some OSs try fairness
 Some implement Quality Of Service (i.e. IPQOS)
 Buffering - store data in memory while transferring between devices
 To cope with device speed mismatch
 To cope with device transfer size mismatch
 To maintain “copy semantics”
 Double buffering – two copies of the data
 Kernel and user
 Varying sizes
 Full / being processed and not-full / being used
 Copy-on-write can be used for efficiency in some cases

Device-status Table

Sun Enterprise 6000 Device-Transfer Rates

Kernel I/O Subsystem
 Caching - faster device holding copy of data
 Always just a copy
 Key to performance
 Sometimes combined with buffering
 Spooling - hold output for a device
 If device can serve only one request at a time
 i.e., Printing
 Device reservation - provides exclusive access to a device
 System calls for allocation and de-allocation
 Watch out for deadlock

Error Handling
 OS can recover from disk read, device unavailable, transient
write failures
 Retry a read or write, for example
 Some systems more advanced – Solaris FMA, AIX
 Track error frequencies, stop using device with
increasing frequency of retry-able errors
 Most return an error number or code when I/O request fails
 System error logs hold problem reports

I/O Protection
 User process may accidentally or purposefully attempt to
disrupt normal operation via illegal I/O instructions
 All I/O instructions defined to be privileged
 I/O must be performed via system calls
 Memory-mapped and I/O port memory locations must
be protected too

Use of a System Call to Perform I/O

Kernel Data Structures
 Kernel keeps state info for I/O components, including open file
tables, network connections, character device state
 Many, many complex data structures to track buffers, memory
allocation, “dirty” blocks
 Some use object-oriented methods and message passing to
implement I/O
 Windows uses message passing
 Message with I/O information passed from user mode
into kernel
 Message modified as it flows through to device driver
and back to process
 Pros / cons?

UNIX I/O Kernel Structure

Power Management
 Not strictly domain of I/O, but much is I/O related
 Computers and devices use electricity, generate heat, frequently
require cooling
 OSes can help manage and improve use
 Cloud computing environments move virtual machines
between servers
 Can end up evacuating whole systems and shutting them
down
 Mobile computing has power management as first class OS
aspect

Power Management (Cont.)
 For example, Android implements
 Component-level power management
 Understands relationship between components
 Build device tree representing physical device topology
 System bus -> I/O subsystem -> {flash, USB storage}
 Device driver tracks state of device, whether in use
 Unused component – turn it off
 All devices in tree branch unused – turn off branch
 Wake locks – like other locks but prevent sleep of device when lock
is held
 Power collapse – put a device into very deep sleep
 Marginal power use
 Only awake enough to respond to external stimuli (button
press, incoming call)

I/O Requests to Hardware Operations
 Consider reading a file from disk for a process:
 Determine device holding file
 Translate name to device representation
 Physically read data from disk into buffer
 Make data available to requesting process
 Return control to process

Life Cycle of An I/O Request

STREAMS
 STREAM – a full-duplex communication channel between a
user-level process and a device in Unix System V and beyond
 A STREAM consists of:
 STREAM head interfaces with the user process
 driver end interfaces with the device
 zero or more STREAM modules between them
 Each module contains a read queue and a write queue
 Message passing is used to communicate between queues
 Flow control option to indicate available or busy
 Asynchronous internally, synchronous where user process
communicates with stream head

The STREAMS Structure

Performance
 I/O a major factor in system performance:
 Demands CPU to execute device driver, kernel I/O
code
 Context switches due to interrupts
 Data copying
 Network traffic especially stressful

Intercomputer Communications

Improving Performance
 Reduce number of context switches
 Reduce data copying
 Reduce interrupts by using large transfers, smart controllers,
polling
 Use DMA
 Use smarter hardware devices
 Balance CPU, memory, bus, and I/O performance for highest
throughput
 Move user-mode processes / daemons to kernel threads

Device-Functionality Progression

End of Chapter 13

cs8493 - operating systems unit 4

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cs8493 - operating systems unit 4