Operating System Concepts
chapter 12
CS 355
Operating Systems
Dr. Matthew Wright
Background: Magnetic Disks
• Rotate 60 to 200 times per second
• Transfer rate: rate at which data flows between drive and computer
• Positioning time (random-access time): time to move disk arm to desired
cylinder (seek time) and time for desired sector to rotate under the disk head
(rotational latency)
Disk Address Structure
• Disks are addressed as a large 1-dimensional array of logical blocks (usually
512 bytes per logical block).
• This array is mapped onto the sectors of the disk, usually with sector 0 on
the outermost cylinder, then through that track, then through that cylinder,
and then through the other cylinders working toward the center of the disk.
• Converting logical addresses to cylinder and track numbers is difficult
because:
– Most disks have some defective sectors, which are replaced by spare
sectors elsewhere on the disk.
– The number of sectors per track might not be constant.
• Constant linear velocity (CLV): tracks farther from center hold more bits, so
disk rotates faster when reading these tracks to keep data rate constant
(CSs, DVDs commonly use this method)
• Constant angular velocity (CAV): rotational speed is constant, so bit density
decreases from inner tracks to outer tracks to keep data rate constant
Disk Scheduling: FCFS
• Simple, but generally doesn’t provide the fastest service
• Example: suppose the read/write heads start on cylinder 53, and disk
queue has requests for I/O to blocks on the following cylinders:
98, 183, 37, 122, 14, 124, 65, 67
Diagram shows
read/write head
movement to
service the
requests FCFS.
Total head
movement spans
640 cylinders.
Disk Scheduling: SSTF
• Shortest Seek Time First (SSTF): service the requests closest to the
current position of the read/write heads
• This is similar to SJF scheduling, and could starve some requests.
• Example: heads at cylinder 53; disk request queue contains:
98, 183, 37, 122, 14, 124, 65, 67
Diagram shows
read/write head
movement to
service the
requests SSTF.
Total head
movement spans
236 cylinders.
Disk Scheduling: SCAN
• SCAN algorithm: disk heads start at one end, move towards the
other end, then return, servicing requests along each way
• Example: heads at cylinder 53 moving toward 0; request queue:
98, 183, 37, 122, 14, 124, 65, 67
Diagram shows
read/write head
movement to
service the
requests with SCAN
algorithm.
Total head
movement spans
236 cylinders.
Disk Scheduling: C-SCAN
• Circular SCAN (C-SCAN): Disk heads start at one end, move towards the
other end, servicing requests along each way. Disk heads return
immediately to the first end without servicing requests, then repeat.
• Example: heads at cylinder 53; request queue:
98, 183, 37, 122, 14, 124, 65, 67
Diagram shows
read/write head
movement to
service the
requests with CSCAN algorithm.
Total head
movement spans
383 cylinders.
Disk Scheduling: LOOK and C-LOOK
• Like SPAN or C-SPAN algorithms, but only going as far as the last
request in either direction.
• Example: heads at cylinder 53; request queue:
98, 183, 37, 122, 14, 124, 65, 67
Diagram shows
read/write head
movement to
service the
requests with CSCAN algorithm.
Total head
movement spans
322 cylinders.
Selecting a Disk-Scheduling Algorithm
• Which algorithm to choose?
– SSTF is common and better than FCFS.
– SCAN and C-SCAN perform better for systems that place a heavy load on
the disk.
– Performance depends on the number and types of requests, and the fileallocation method.
– In general, either SSTF or LOOK is a reasonable choice for the default
algorithm.
• 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.
• Why not let the controller built into the disk hardware manage the
scheduling?
– The disk hardware can take into account both seek time and rotational
latency.
– The OS may choose to mandate the disk scheduling to guarantee priority
of certain types of I/O.
Disk Management
• The Operating System may also be responsible for tasks such as disk
formatting, booting from disk, and bad-block recovery.
• Low-level formatting divides a disk into sectors, and is usually
performed when the disk is manufactured.
• Logical formatting creates a file system on the disk, and is done by the
OS.
• The OS maintains the boot blocks (or boot partition) that contain the
bootstrap loader.
• Bad blocks: disk blocks may fail
– An error-correcting code (ECC) stored with each block can detect and
possibly correct an error (if so, it is called a soft error).
– Disks contain spare sectors which are substituted for bad sectors.
– If the system cannot recover from the error, it is called a hard error,
and manual intervention may be required.
Swap-Space Management
• Recall that memory uses disk space as an extension of main
memory; this disk space is called the swap space, even for systems
that implement paging rather than pure swapping.
• Swap-space can be:
– A file in the normal file system: easy to implement, but slow in
practice
– A separate (raw) disk partition: requires a swap-space manager,
but can be optimized for speed rather than storage efficiency
• Linux allows the administrator to choose whether the swap space
is in a file or in a raw disk partition.
RAID Structure
• RAID: Redundant Array of Independent Disks or Redundant Array of
Inexpensive Disks
• In systems with large numbers of disks, disk failures are common.
• Redundancy allows the recovery of data when disk(s) fail.
– Mirroring: A logical disk consists of two physical disks, and every write
is carried out on both disks.
– Bit-level striping: Splits the bits of each byte across multiple disks,
which improves the transfer rate.
– Block-level striping: Splits blocks of a file across multiple disks, which
improves the access rate for large files and allows for concurrent reads
of small files.
– A nonvolatile RAM (NVRAM) cache can be used to protect data
waiting to be written in case a power failure occurs.
• Are disk failures really independent?
• What if multiple disks fail simultaneously?
RAID Levels
• RAID level 0: non-redundant striping
– Data striped at the block level, with no
redundancy.
• RAID level 1: mirrored disks
– Two copies of data stored on different disks.
– Data not striped.
– Easy to recover data from one disk that fails
• RAID 0 + 1: combines RAID levels 0 and 1
– Provides both
performance and
reliability.
RAID Levels
• RAID level 2: error-correcting codes
– Data striped across disks at the bit level.
– Disks labeled P store extra bits that can be used
to reconstruct data if one disk fails.
– Requires fewer disks than RAID level 1.
– Requires computation of the error-correction
bits at every write, and failure recovery requires
lots of reads and computation.
RAID Levels
• RAID level 3: bit-interleaved parity
– Data striped across disks at the bit level.
– Since disk controllers can detect whether a
sector has read correctly, a single parity bit can
be used for error detection and correction.
– As good as RAID level 2 in practice, but less
expensive.
– Still requires extra computation for parity bits.
• RAID level 4: block-interleaved parity
– Data striped across disks at the block level.
– Stores parity blocks on a separate disk, which
can be used to reconstruct the blocks on a single
failed disk.
RAID Levels
• RAID level 5: block-interleaved distributed parity
– Data striped across disks at the
block level.
– Spreads data and parity blocks across all disks.
– Avoids possible overuse of a single parity disk, which could happen
with RAID level 4.
• RAID level 6: P + Q redundancy scheme
– Like RAID level 5, but stores extra
redundant information to guard
against simultaneous failures of
multiple disks.
– Uses error-correcting codes such as Reed-Solomon codes.
RAID Implementation
• RAID can be implemented at various levels:
– At the kernel of system software level
– By the host bus-adapter hardware
– By storage array hardware
– In the Storage Area Network (SAN) by disk virtualization devices
• Some RAID implementations include a hot spare: an extra disk that
is not used until one disk fails, at which time the system
automatically restores data onto the spare disk.
Stable-Storage Implementation
• Stable storage: storage that never loses stored information.
• Write-ahead logging (used to implement atomic transactions) requires
stable storage.
• 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 failure
during an update will not leave all copies in a damaged state, and so
that we can safely recover from a failure.
• Three possible outcomes of a disk write:
1. Successful completion: all of the data written successfully
2. Partial failure: only some of the data written successfully
3. Total failure: occurs before write starts; previous data remains intact
Stable-Storage Implementation
• Strategy: maintain two (identical) physical blocks for each logical block, on
different disks, with error-detection bits for each block
• A write operation proceeds as:
– Write the information to the first physical block.
– When the first write completes, then write the same information to the
second physical block.
– When the second write completes, then declare the operation successful.
• During failure recovery, examine each pair of physical blocks:
– If both are the same and neither contains a detectable error, then do
nothing.
– If one block contains a detectable error, then replace its contents with the
other block.
– If neither block contains a detectable error, but the values differ, then
replace the contents of the first block with that of the second.
• As long as both copies don’t fail simultaneously, we guarantee that a write
operation will either succeed completely or result in no change.
Tertiary Storage
• Most OSs handle removable disks almost exactly like fixed disks — a new
cartridge is formatted and an empty file system is generated on the disk.
• Tapes are presented as a raw storage medium, i.e., and application does not
open a file on the tape, it opens the whole tape drive as a raw device.
– Usually the tape drive is reserved for the exclusive use of that application.
– Since the OS does not provide file system services, the application must
decide how to use the array of blocks.
– Since every application makes up its own rules for how to organize a tape, a
tape full of data can generally only be used by the program that created it.
• The issue of naming files on removable media is especially difficult when we
want to write data on a removable cartridge on one computer, and then use the
cartridge in another computer.
• Contemporary OSs generally leave the name space problem unsolved for
removable media, and depend on applications and users to figure out how to
access and interpret the data.
• Some kinds of removable media (e.g., CDs) are so well standardized that all
computers use them the same way.