Operating system presentation part 2 2025

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

10.2 Silberschatz, Galvin and Gagne ©2013
Edition
Unit-4 part-2
 Secondary –Storage Structure: Overview of Mass-
Storage Structure, Disk Structure, Disk Attachment,
RAID Structure, Stable-Storage Implementation,
Tertiary-Storage Structure.

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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

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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

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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

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Hard disk

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Moving-head Disk Mechanism

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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)

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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

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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

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Magnetic Tape
 Magnetic disks provide the bulk of secondary storage for modern
computer systems.
 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

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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

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Disk Attachment
 Computers access disk storage in two ways. One way is via I/O
ports (or host-attached storage) this is common on small systems.
 The other way is via a remote host in a distributed file system; this
is referred to as Network-attached storage
 Host-attached storage accessed through I/O ports talking to I/O
busses
 FC(Fibre channel) 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

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Host-Attached Storage
 Host-attached storage is storage accessed through local I/0
ports. These ports use several technologies. The typical
desktop PC uses an I/0 bus architecture called IDE or ATA..
 A newer, similar protocol that has simplified cabling is SATA.
High-end workstations and servers generally use more
sophisticated I/0 architectures, such as SCSI and fiber
channel (FC).
 A wide variety of storage devices are suitable for use as
host-attached storage. Among these are hard disk drives,
RAID arrays, and CD, DVD, and tape drives.

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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

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Network-Attached Storage
 A network-attached storage (NAS) device is a special-purpose
storage system that is accessed remotely over a data network
 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)

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Storage Area Network
 One drawback of network-attached storage systems is that the storage
I/O operations consume bandwidth on the data network, thereby
increasing the latency of network communication
 SANs are typically composed of hosts, switches, storage elements,
and storage devices that are interconnected using a variety of
technologies, topologies, and protocols.
 Common in large storage environments
 Multiple hosts attached to multiple storage arrays - flexible

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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
 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

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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

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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.
 Access time has two major components
 Seek time is the time for the disk are to move the heads to the
cylinder containing the desired sector.
 Rotational latency is the additional time waiting for the disk to rotate
the desired sector to the disk head.
 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.
21

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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

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Disk Scheduling
 There are many Disk Scheduling Algorithms but before discussing them
let’s have a quick look at some of the important terms:
 Seek Time:Seek time is the time taken to locate the disk arm to a specified
track where the data is to be read or write. So the disk scheduling algorithm
that gives minimum average seek time is better.
 Rotational Latency: Rotational Latency is the time taken by the desired
sector of disk to rotate into a position so that it can access the read/write
heads. So the disk scheduling algorithm that gives minimum rotational
latency is better.
 Transfer Time: Transfer time is the time to transfer the data. It depends on
the rotating speed of the disk and number of bytes to be transferred.
 Disk Access Time: Disk Access Time is
Disk Access Time = Seek Time + Rotational Latency + Transfer Time

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Disk Scheduling

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FCFS
 FCFS: Fisrt Come First Serve
 FCFS is the simplest of all the Disk Scheduling
Algorithms. In FCFS, the requests are addressed in the
order they arrive in the disk queue. Let us understand
this with the help of an example 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

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FCFS
Illustration shows total head movement of 640 cylinders
If the disk
head is
initially at
cylinder 53, it
will first move
from
53 to 98, then
to 183, 37,
122, 14, 124,
65, and finally
to 67, for a
total head
movement of
640 cylinders.

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FCFS
 Advantages:
 Every request gets a fair chance
 No indefinite postponement
 Disadvantages:
 Does not try to optimize seek time
 May not provide the best possible service

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SSTF
 SSTF:
In SSTF (Shortest Seek Time First), requests having
shortest seek time are executed first. So, the seek time
of every request is calculated in advance in the queue
and then they are scheduled according to their
calculated seek time. As a result, the request near the
disk arm will get executed first. SSTF is certainly an
improvement over FCFS as it decreases the average
response time and increases the throughput of system.
Let us understand this with the help of an example

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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

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SSTF (contd..)
For our example request queue, the closest request to the initial
head
position (53) is at cylinder 65. Once we are at cylinder 65, the
next closest request is at cylinder 67.
From there, the request at cylinder 37 is closer than the one at
98, so 37 is served next.
Continuing, we service the request at cylinder 14,then 98, 122,
124, and finally 183 (Figure 12.5). This scheduling method
results in a total head movement of only 236 cylinders
Advantages:
•Average Response Time decreases
•Throughput increases
Disadvantages:
•Overhead to calculate seek time in advance
•Can cause Starvation for a request if it has higher seek
time as compared to incoming requests
•High variance of response time as SSTF favors only some
requests

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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

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SCAN(contd..)
 Let's return to our example to illustrate. Before applying SCAN
to schedule the requests on cylinders 98, 183,37, 122, 14, 124,
65, and 67, we need to know the direction of head movement in
addition to the head's current position. Assuming that the disk
arm is moving toward 0 and that the initial head position is
again 53, the head will next service 37 and then 14.
 At cylinder 0, the arm will reverse and will move toward the
other end of the disk, servicing the requests at 65, 67, 98, 122,
124, and 183 (Figure 12.6). If a request arrives in the queue
just in front of the head, it will be serviced almost immediately;
are request arriving just behind the head will have to wait until
the arm moves to the end of the disk, reverses direction, and
comes back.

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SCAN (Cont.)

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SCAN
 Advantages:
 High throughput
 Low variance of response time
 Average response time
 Disadvantages:
 Long waiting time for requests for
locations just visited by disk arm

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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
 It is a variant of SCAN designed to provide a more uniform wait time.
Like SCAN, C-SCAN moves the head from one end of the disk to the
other, servicing requests along the way. When the head reaches the
other end, however, it immediately returns to the beginning of the disk
without servicing any requests on the return trip (Figure 12.7).
 Advantages:
 Provides more uniform wait time compared to SCAN

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C-SCAN (Cont.)

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LOOK
 LOOK: It is similar to the SCAN disk
scheduling algorithm except for the
difference that the disk arm in spite of
going to the end of the disk goes only to
the last request to be serviced in front of
the head and then reverses its direction
from there only. Thus it prevents the extra
delay which occurred due to unnecessary
traversal to the end of the disk.

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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
 CLOOK: As LOOK is similar to SCAN algorithm, in similar
way, CLOOK is similar to CSCAN disk scheduling algorithm. In
CLOOK, the disk arm in spite of going to the end goes only to
the last request to be serviced in front of the head and then
from there goes to the other end’s last request. Thus, it also
prevents the extra delay which occurred due to unnecessary
traversal to the end of the disk

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C-LOOK (Cont.)

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Selecting a Disk-Scheduling Algorithm
 Given so many disk-scheduling algorithmsf how do we choose the best one?
 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 queuing effect OS queue ordering efforts?

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 The scheduling algorithms described here
consider only the seek distances. For
modern disks, the rotational latency can
be nearly as large as the average seek
time. It is difficult for the operating system
to schedule for improved rotational
latency, though, because modern disks do
not disclose the physical location of logical
blocks

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 Disk manufacturers have been alleviating
this problem by implementing disk-
scheduling algorithms in the controller
hardware built into the disk drive. If the
operating system sends a batch of
requests to the controller, the controller
an queue them and then schedule them to
improve both the seek time and the
rotational latency.

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Disk Management
 The operating system is responsible for several other aspects of disk
management, too. Here we discuss disk initialization, booting from disk, and
bad-block recovery.
 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

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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

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Booting from a Disk in Windows

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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

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Data Structures for Swapping on Linux Systems

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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

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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

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RAID Levels

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RAID Level-2
 Each byte in a memory system may have a parity bit associated
with it that records whether the number of bits in the byte set to 1
is even (parity =0) or odd (parity =1). If one of the bits in the byte
is damaged (either a 1 becomes a 0, or a 0 becomes a 1), the
parity of the byte changes and thus will not match the stored
parity. Similarly, if the stored parity bit is damaged, it will not
match the computed parity. Thus, all single-bit errors are
detected by the memory system.
 RAID Level-3 :disk controllers can detect whether a sector has
been read correctly, so a single parity bit can be used for error
correction as well as for detection. The idea is as follows: If one
of the sectors is damaged, we know exactly which sector it is,
and we can figure out whether any bit in the sector is a 1 or a 0
by computing the parity of the corresponding bits from sectors in
the other disks. If the parity of the remaining bits is equal to the
stored parity, the missing bit is 0; otherwise, it is 1.

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RAID level 3
 RAID level 3 has two advantages over level 1. First, the
storage overhead is reduced because only one parity
disk is needed for several regular disks, whereas one
mirror disk is needed for every disk in level 1.
 Second, since reads and writes of a byte are spread out
over multiple disks with N-way striping of data, the
transfer rate for reading or writing a single block is N
times as fast as with RAID level 1.

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RAID Level 4
 RAID Level 4. RAID level 4, or block-interleaved parity
organization, uses block-level striping, as in RAID 0, and
in addition keeps a parity block on a separate disk for
corresponding blocks from N other disks. This scheme is
diagramed in Figure 12.11(e).
 If one of the disks fails, the parity block can be used with
the corresponding blocks from the other disks to restore
the blocks of the failed disk.
 A block read accesses only one disk, allowing other
requests to be processed by the other disks.

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RAID Level 5
 RAID Level 5. RAID level 5, or block-interleaved distributed parity,
differs from level 4 by spreading data and parity among all N + 1
disks, rather than storing data in N disks and parity in one disk.
 For each block, one of the disks stores the parity, and the others
store data. For example, with an array of five disks,
 Figure 12.11(f), where the Ps are distributed across all the disks. A
parity block cannot store parity for blocks in the same disk,
 Because a disk failure would result in loss of data as well as of
parity, and hence the loss would not be recoverable. By spreading
the parity across all the disks in the set, RAID 5 avoids the potential
overuse of a single parity disk that can occur with RAID 4. RAID 5 is
the most common parity RAID system

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RAID Level 6
 RAID Level 6. IAID level 6, also called the P + Q
redundancy scheme, is much like RAID level 5 but
stores extra redundant information to guard against
multiple disk failures. Instead of parity, error-correcting
codes such as the Reed-Solomon codes are used.
 In the scheme shown in Figure 12.11(g), 2 bits of
redundant data are stored for every 4 bits of data
compared with 1 parity bit in level 5-and the system can
tolerate two disk failures.

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RAID (0 + 1) and (1 + 0)
RAID Level 0 +
1. RAID level 0
+ 1 refers to a
combination of
RAID levels
0 and 1. RAID
0 provides the
performance,
while RAID 1
provides the
reliability.
Generally, this
level provides
better
performance
than RAID 5.
RAID
level 1 + 0, in which
disks are mirrored in
pairs, and then the
resulting
mirror pairs are striped.
This RAID has some
theoretical advantages
over
RAID 0 + 1

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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

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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

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ZFS Checksums All Metadata and Data

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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

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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

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Tertiary-Storage Structure
 Because cost is so important, in practice, tertiary storage
is built with removable media.
 The most common examples are floppy disks, tapes,
and read-only, write-once, and rewritable CDs and
DVDs.
 Many any other kinds of tertiary storage devices are
available as well, including removable devices that store
data in flash memory and interact with the computer
system via a USB interface.

Operating system presentation part 2 2025

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