Database Lecture12-Secondary Storage-PA.ppt

Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th
Edition
Chapter 12: Disk Structure

12.2 Silberschatz, Galvin and Gagne ©2013
Edition
Moving-head Disk Mechanism

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Hard Disks
 Range from 30GB to 3TB per drive
 Seek Time: The time taken for a disk drive to
locate the area on the disk where the data to be
read is stored. (to particular track)
 Seek time from 3ms to 12ms – 9ms common
for desktop drives
 Seek time is normally expressed in
milliseconds (commonly abbreviated
"msec" or "ms"), with average seek times
for most modern drives today in a rather
tight range of 8 to 10 ms.
 Latency Time: The time taken by head to
access the required sector is known as
latency.
 Latency based on spindle speed
 1 / (RPM / 60) = 60 / RPM
(From Wikipedia)

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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 (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 (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

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FCFS
Illustration shows total head movement of 640 cylinders
45+85+146+85+108+110+59+2

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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
12+2+30+23+84+24+2+59

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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 208 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 (Cont.)

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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
 Total number of cylinders?

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C-SCAN (Cont.)

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LOOK/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?
 It is called LOOK because they look for a request before
continuing to move in a given direction.

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C-LOOK (Cont.)

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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
 Algorithms only consider seek distances, What about rotational latency?
 Difficult for OS to calculate because modern disks do not disclose
the physical location of logical blocks.

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Disk Management
It includes the following tasks
Low-level formatting
Logical formatting
Booting
Bad block recovery
 Swap space

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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
 When the controller writes a sector of data during normal I/O, the ECC is
updated with the value calculated from all the bytes in the data area. When the
sector is read, the ECC is recalculated and compared with stored value. If
mismatched, this means data is corrupted and the disk’s sector is bad.
 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
 Disks have moving parts and small tolerances, they are prone to failure.
Sometimes need backup media.
 Most disks even come from factory with bad blocks.
 E.g. MS-DOS formats and finds bad blocks. If found, it writes a special value into
the corresponding FAT entry to tell the allocation routine not to use it.
 More sophisticated disks like SCSI, maintains a list of bad blocks on the disk
and updated over the life of the disks. Low level formatting also set aside spare
sectors not visible to OS. The controller can be told to replace each bad sector
logically with one of the spare sectors called sector sparing .

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Booting from a Disk in Windows
Master Boot Record

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21
Bad Blocks
 Disk sectors (blocks) may become defective. Can no longer store data.
 Hardware defect
 System should not put data there.
 Possible Strategy:
 A bad block X can be remapped to a good block Y
 Whenever OS tries to access X, disk controller accesses Y.
 Some sectors (blocks) of disk can be reserved for this mapping.
 This is called sector sparing.

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22
Bad Blocks
x
z
y w Disk
Disk Controller
x y
z w
bad block mapping table
block (sector) request
bad sector
Table stored
on disk
spare sectors

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Bad Blocks
 Disk is having moving part as well as small tolerances, so prone to failure.
 To check for bad blocks, program ‘chkdsk’ can be used.
 In SCSI disks, ‘Sector Sparing’ method is used to handle with bad blocks.
Steps:
1) Find logical block 10
2) Calculates ECC and found it is bad block and send OS a message.
3) Reboot the system and tell SCSI controller to replace the block.
4) So whenever , 10th
block is referred automatically control goes to
replaced spare sector.
 Another method is ‘Sector Slipping’

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Disk Management (Cont.)
 Alternative to sector sparing is Sector Slipping.
 Methods such as sector sparing or sector slipping is used to handle bad blocks.
 Under sector sparing, a new sector is allocated to replace a bad sector.
 Under sector slipping: for example, if sector 20 becomes defective, and the first
available sector follows sector 202, then all sectors from 20 to 202 are remapped
moving them down one spot.
SECTOR SLIPPING: The Process of moving all the sectors down one position from
the bad sector is called as sector slipping.
Example: If the logical block 17 becomes defective and the first available spare
follows sector 202. Sector slipping then remaps all the sectors from 17 to 202,
moving them all down one spot. That is, sector 202 is copied into the spare, then
sector 201 into 202, then 200 into 201, and so on, until sector 18 is copied into
sector 19. Slipping the sectors in this way frees up the space of sector 18 so that
sector 17 can be mapped to it.

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25
Disk Management
 Low-level formatting, or physical formatting — Dividing a disk into sectors
that the disk controller can read and write.
 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.
 Logical formatting or “making a file system”.

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26
Low Level Formatting
Sector Sector Sector ….
magnetic material that can store bits
Disk before low level formatting
Disk after low level formatting
HDR ECC
Data (512 bytes)
sector number error correcting code

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27
Boot Process
Tiny
Boot
program
ROM
RAM Disk
MBR
Boot Block
CPU
partition1
partition2
partition3
boot codepartition table
1. Boot code in
ROM is run; it brings
MBR into memory
and starts MBR boot
code
2. MBR boot code
runs; looks
to partition table;
learns about
the boot partition;
brings and starts the
boot code in the boot
partition
3. Boot code in boot
partition loads
the kernel sitting
in that partition
Power ON
kernel

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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
 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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 Swap space in Linux is used when the amount of physical memory
(RAM) is full. If the system needs more memory resources and the
RAM is full, inactive pages in memory are moved to the swap
space. While swap space can help machines with a small amount of
RAM, it should not be considered a replacement for more RAM.
Swap space is located on hard drives, which have a slower access
time than physical memory.
Swap-Space Management Cont…

Edition
 Since disk access is much slower than memory access, using swap space
significantly decreases system performance.
 Main goal of swap space is to provide the best throughput for the virtual
memory system.
Swap Space Use: Different ways
 Use swap space to hold the entire process image eg code, data. Paging
system may simply store pages that have been pushed out of main
memory.
 Some OS like UNIX, allow multiple swap spaces. These are put on
separate disks so that load placed on the I/O system by paging and
swapping can be spread over the system.
 It is safer to overestimate than to underestimate swap space, because if a
system runs out of swap space it may be forced to abort processes or may
crash entirely.

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Swap Space Location: can reside in two places
Can be carved out of the normal file system or
Can be in a separate disk partition
1.If normal file system, then can be used to create it, name it and allocate its
space. Easy approach but inefficient. Navigation of files and disk will take
time, extra disk access. External fragmentation will increase swapping time.
2.Separate swap space management is used to allocate and deallocate the
blocks. Good speed than storage efficiency. Internal fragmentation may
increase but acceptable as data in swap space is stored for shorter duration
and swap space area can be accessed much more efficiently. Swap space is
fixed during partition and if need to change then repartitioning is required.
3.Some OS like Solaris 2 is flexible and can use any of the method
depending upon need.

Database Lecture12-Secondary Storage-PA.ppt

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