netw 3005 mass storage (how discs work). notice i was unaware just how much was missing in the...

NETW 3005

Mass Storage(How discs work)

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.

NETW3005 (Operating Systems) Lecture 11 - How discs work/44 2

Reading

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


Last lecture: I/O systems

• Hardware: ports, buses, controllers

• Application I/O interface

• Kernel I/O services


This lecture: How disks work

• The mechanics of disks

• Disk scheduling

• Formatting and booting

• Disk reliability: bad blocks, RAIDs

• Swap space management


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.


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.


The mechanics of disks


Addressing

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


Cylinders are vertically formed by tracks. In other words, track 12 on platter 0 plus track 12 on platter 1 etc. is cylinder 12. The number of cylinders of a disk drive exactly equals the number of tracks on a single surface in the drive.

Access time has two major components

•Seek time is the time taken to move the heads to the cylinder containing the desired sector.

•Rotational latency is the time taken to rotate the desired sector to the disk head.

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.


Disk scheduling

• kernel’s I/O subsystem schedules pool of pending I/O requests .

• Imagine a queue of I/O requests to a given disk.

• Ordering these requests in different ways will result in different head seek times.


Algorithms for disk scheduling• Criteria for evaluating algorithms:

– Seek time

– Fairness (in particular, starvation)


1. FCFS

2. Shortest seek-time first scheduling

3. SCAN

4. C SCAN

5. C LOOK

FCFS scheduling

• Treat I/O requests in FCFS order.


Calculation of the seek time for the schedule givenon 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 numberof cylinders' the head is moving – that gives us the distancewhich is directly proportional to seek timei.e if the distance is increasing seek time is also increasing


Illustration shows total head movement of 640 cylinders.

Advantages and disadvantages?

• Advantages?– no starvation: every request is serviced – Simple to implement.

• Disadvantages?

– big swing in head seek.


Shortest seek-time first scheduling

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



• Advantages?– You get much shorter seek times this way,

because you’re eliminating the big swings. (At least, you’ll only get them if there’s nothing closer.)

• Disadvantages?– Starvation is a possibility.


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 some many cylinder movements not time

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.


•(53 - 37) + (37 - 14) + (65 - 14) + (67 - 65) + •(98 - 67) + (122 - 98) + (124 - 122) + (183 - 124)

•= 208


• Advantages?– Avoids starvation

• Disadvantages?– Requests for the middle of the disk are

advantaged over those at the ends.


C-SCAN scheduling

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



C-SCAN scheduling


• Advantages?– No region of the disk are favored.

• Disadvantages?

requires one long seek after finished going up

have to go back to the beginning


C-SCAN scheduling

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.



• Advantages?– No region of the disk are favored

• Disadvantages?

more inclined to serve the middle cylinder requests


Comparison of Disk Scheduling Algorithms

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.


Selecting a Disk-Scheduling Algorithm • FCFS- Ideal for LOW Load Disk Schedule.

• SSTF – ideal for Linked and indexed allocation technique to reduce the head seek time.

• SCAN and C-SCAN- ideal for heavy load on the disk.

• C – scan is ideal for contiguous allocation technique

• Either SSTF or LOOK reasonable choice for the default algorithm.

Where should index blocks be stored?

• Near the blocks containing the file’s data.


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.

Disk formatting• Low-level formatting: normally done in

the factory.

• Creates the sectors on the disk, and fills 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).


Header

Sector No

Data ( 512 bytes) Trailer( ECC)

Disk formatting

• Partitioning: done by the operating system.

• Logical formatting: making an (empty) file system.


File System

NTFS FAT 32

Bad blocks• A bad sector is a sector on a computer's disk

drive that cannot be used due to permanent damage .


Bad blocks are blocked in the FAT – table

The Controller maintains list of bad blocks and spare blocks right from Low Level formatting .

Controller can replace each bad sector with one of the spare sector

Problems with sparing?• Sector sparing could invalidate any

optimization by the operating systems disk scheduling algorithm


•Sector slipping: shuffling all the data on disk to make room for the spare block right next to the one it’s replacing.

•Provision of spare sectors in each cylinders

Error recovery and RAIDs• RAID is the organization of multiple disks

into a large, high performance logical disk.

• Each block of data is broken into sub-blocks, with one sub-block stored on each disk.


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.


10011100 01101100 11110000

11110000 10011100 =01101100

Primary Boot

strap loader

Secondary boot loader

LOADS THE OS INTO THE MAIN

MEMORYLOADS

Booting a system

Swap space management•Swap space holds entire process or page or segment which

Is swapped out to the backing store from main memory

Swap space implementation

File Systemswap space is simply

a large file within file system

Special raw partition swap space

Is Blocks in the raw partition

Navigating takes moreTime

Speed of access is better than file system

•Some OSs can swap in both file space and raw swap partitions, e.g. Solaris 2.

Next Lecture

!Revision!Make sure you come along(Exam hints are possible)

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 it’s 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


1 xor 1 = 0 1 xor 0 = 1 0 xor 0 = 0 1110 xor 1001 = 0111

7 bits of data(number of 1s)

8 bits including parity

even odd

0000000 (0) 00000000 10000000

1010001 (3) 11010001 01010001

1101001 (4) 01101001 11101001

1111111 (7) 11111111 01111111

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.


Swap-space management

• 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?


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.


netw 3005 mass storage (how discs work). notice i was unaware just how much was missing in the...

Documents

discs work44this lecture

discs work44last lecture

discs work44algorithms

discs work44noticei

discs work44readingfor

discs work44access time

lecture slides

disk head