COSC 243

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NETW 3005
Mass Storage (How discs work)
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Notice

• I was unaware just how much was missing in the
  printed notes.
• As a partial remedy for that, the lecture slides
  will appear on the web within the next week.

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Reading

• For this lecture, you should have read Chapter 12
  (Sections 1-4).

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Last lecture I/O systems

• Hardware ports, buses, controllers
• Application I/O interface
• Kernel I/O services

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This lecture How disks work

• The mechanics of disks
• Disk scheduling
• Formatting and booting
• Disk reliability bad blocks, RAIDs
• Swap space management

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The mechanics of disks

• A disk drive consists of
• a number of platters
• with two surfaces each
• arranged on a rotating spindle
• with a head for each surface.

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Some terminology

• A single disk platter has two surfaces.
• Each surface is organised into concentric circles
  called tracks.
• All tracks of the same radius form a cylinder.
• Each track is divided into sectors.
• A sector is the smallest addressable part of the
  disk.

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The mechanics of disks
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Addressing

• Information on the disk is referenced by a
  multi-part address which includes
• drive number,
• cylinder number,
• surface number and,
• sector number.

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More terminology

• The heads all move together, accessing a cylinder
  of the disk.
• To access a particular sector, the heads are
  moved to the appropriate cylinder, the correct
  head is enabled, and the head waits until the
  correct sector comes under it.

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Disk scheduling

• As we have seen, one of the bene?ts of the
  kernels I/O subsystem is that a pool of pending
  I/O requests can be scheduled.
• Imagine a queue of I/O requests to a given disk.
• Ordering these requests in different ways will
  result in different head seek times.

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Algorithms for disk scheduling

• Criteria for evaluating algorithms
• Seek time
• Fairness (in particular, starvation)

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FCFS scheduling

• Treat I/O requests in FCFS order.

Calculation of the seek time for the schedule
given on the next slide (98 - 53) (183 - 98)
(183 - 37) (122 - 37) (122 - 14) (124 -
14) (124 - 65) (67 - 65) 640 cylinders
Here we do not calculate the time but only the
number of cylinders' the head is moving that
gives us the distance which is directly
proportional to seek time i.e if the distance is
increasing seek time is also increasing
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Illustration shows total head movement of 640
cylinders.
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Advantages and disadvantages?

• Advantages?
• Fair.
• Simple to implement. (Simple also means quick!)
• Disadvantages?
• Theres nothing to stop big swings.

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Shortest seek-time first scheduling

• At any moment, choose the request with the
  shortest distance from the current head position.

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Illustration shows total head movement of 236
cylinders.
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Advantages and disadvantages?
Seek time for SSTF is calculated as follows (65
- 53) (67 - 65) (67 - 37) (37 - 14) (98 -
14) (122 - 98) (124 - 122) (183 - 124)
236 this is so many cylinder movements not time

• Advantages?
• You get much shorter seek times this way, because
  youre eliminating the big swings. (At least,
  youll only get them if theres nothing closer.)
• Disadvantages?
• Starvation is a possibility.

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Is SSTF optimal?

• No it is too short-sighted, i.e. no look-ahead.
• It is possible to develop an optimal algorithm,
  but the time taken to calculate it means its not
  really worth it.
• For example if the head moves to 37 then 14 and
  then 65, 67 and so on seek time will be less
• (53 - 37) (37 - 14) (65 - 14) (67 - 65)
  (98 - 67) (122 - 98) (124 - 122) (183 -
  124) 208

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SCAN scheduling

• Start the disk at one end, and move right to the
  other end, servicing all the I/O requests you get
  to on your way. Then start in the other direction.

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Illustration shows total head movement of 236
cylinders.
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Advantages and disadvantages?
(53 - 37) (37 - 14) (14 - 0) (65 - 0) (67
- 65) (98 - 67) (122 - 98) (124 - 122)
(183 - 124) 236

• Advantages?
• Fairer? Do we prevent starvation?
• Disadvantages?
• Requests for the middle of the disk are
  advantaged over those at the ends.

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C-SCAN scheduling

• Like SCAN, but when the head gets to one end, it
  goes straight to the other end without servicing
  any requests.

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Advantages and disadvantages?

• Advantages?
• We dont favour any region of the disk.
• Disadvantages?
• Starvation is as possible here as in SCAN.

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C-LOOK scheduling

• Like C-SCAN, except that rather than going to the
  ends of the disk, we only go as far as the
  furthest request in each direction.

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Advantages and disadvantages?

• Advantages?
• A definite win over C-SCAN.
• In practice, the C-versions of SCAN/LOOK are
  preferred.
• Disadvantages?
• Starvation, as with SCAN.

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Priority scheduling

• We might not want to treat all these requests as
  equal, e.g. page-fault- generated requests might
  need to be handled first.

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Influences on scheduling algorithms

• The choice of scheduling algorithm depends on how
  the disk is set up to store ?les.

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How much head movement will there be in

• Contiguous allocation?
• head movement will be minimised for sequential
  access, so best will be C-SCAN.
• Linked allocation?
• more head seeks. Makes more sense to use SSTF.
• FATs / indexed allocation?
• here it really makes sense to cache the FAT/index
  block or implement a hot spot.

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Where should index blocks be stored?

• Near the blocks containing the files data.
• Obviously that mightnt be possible.
• But if you have a choice

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Where should directories be stored?

• In the middle of the partition is a good idea, so
  you never have more than half the disk to scan.
• Or near the FAT.
• Best to cache recently-used directory info as
  well.

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Disk formatting

• Low-level formatting normally done in the
  factory.
• Creates the sectors on the disk, and ?lls each
  with an initial data structure
• A header and trailer, containing information used
  by the disk controller - e.g. sector number,
  error-correcting code (ECC).
• A data area (usually 512 bytes).

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Disk formatting

• Partitioning done by the operating system.
• Logical formatting making an (empty) file system.

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Bad blocks

• Sectors on a disk sometimes become defective.
• We need a strategy for preventing these blocks
  being used.
• The usual method is to keep a record of a disks
  bad blocks.

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Strategies

• Simple
• invoke a program periodically to search for bad
  blocks.
• Sophisticated
• Keep a list of bad blocks, and update it when new
  bad blocks are found.
• In either case, set aside a number of spare
  sectors which the O/S cant see, and redirect.

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Problems with sparing?

• They can mess up disk-scheduling algorithms,
  because the O/S doesnt know about the
  redirection.

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Solutions

• Put the scheduling in the controller.
• But that has problems of its own.
• Spread your spare sectors around on the disk.
• Sector slipping shuffling all the data on disk
  to make room for the spare block right next to
  the one its replacing.

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Error recovery and RAIDs

• Modern error-recovery strategies often involve
  cooperation between several disks.
• A frequent cooperation method is a RAID
  (redundant array of independent disks).
• Each block of data is broken into sub-blocks,
  with one sub-block stored on each disk.
• The disks have their rotations synchronised.

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RAIDs

• Mirroring each disk holds all the data.
• Block interleaved parity. A parity bit for the
  group of sub-blocks is written to a special
  parity block. If one of the sub- blocks is lost,
  it can be recovered from the other sub-blocks
  plus the parity block.

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Booting a system

• A computer needs a bootstrap program to run when
  its turned on.
• This program has several tasks
• to initialise CPU registers, main memory
• to load the kernel and start executing it.
• The initial program must be hardwired into the
  computer (typically in ROM).

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Booting a system

• In fact, the initial program is often very small.
• Often it just loads a bigger bootstrap program,
  and this program does the rest.
• The program will be stored at a fixed location on
  the disk, called the boot block.

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Swap-space management

• Recap memory management often uses a backing
  store to hold data from processes being
  multitasked.
• We could implement swap space simply as a file
  within a directory structure.
• Problems with this approach?

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Swap-space management

• A more frequent solution is to create a special
  partition for swap space.
• The disk allocation algorithm on this partition
  is optimised for speed, rather than memory
  efficiency.
• Some OSs can swap in both file space and raw swap
  partitions, e.g. Solaris 2.

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