Lecture 8: Secondary-Storage Structure

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Lecture 8: Secondary-Storage Structure

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Disk Architecture

Cylinder

Track Sector Disk head

3500-7000 rpm

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Disk Structure

Disk drives are addressed as large 1-D arrays of logical blocks, where the logical block is the smallest unit of transfer

The 1-D array of logical blocks is mapped onto the sectors of the disk sequentially

Sector 0 is the 1st sector of the 1st 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

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Disk Structure

Disk

Sector 0 Sector 1Cylinder 0

Cylinder 1

Cylinder 2

Logical view: 1-D array

Physical view

logical block

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Disk Scheduling

The OS is responsible for using hardware efficiently -- for the disk drives, this means having a fast access time and high disk I/O bandwidth

Access time has two major components 1. Seek time: move the head to the

destination cylinder 2. Rotational Latency: time for disk to rotate

the desired sector to the disk head

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Disk Architecture

Cylinder

Track Sector Disk head

3500-7000 rpm

Seek time “seek to the cylinder” = 10 -2 secondLatency time: “Rotate to sector” = 0.5 x (60/6000) ~ 5x10 -3 second

Transmission Time:Transfer bytes intomemory ~ 10 -4 sec(1KB block size/(10MB/s) DMA rate)

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Disk Scheduling

Minimize seek time; seek time seek distance Disk bandwidth= total # of bytes/(time: 1st request to last

request) Some algorithms

1. First-Come-First-Served (FCFS) 2. Shortest-Seek-Time-First (SSTF) 3. SCAN (Elevator algorithm) 4. Circular-SCAN (C-SCAN) 5. C-LOOK (LOOK)

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1. First-Come-First-Served (FCFS)

Queue= 98, 183, 37, 122, 14, 124, 65, 67Head starts at 53

0 14 37 53 65 67 98 122 124 183 1994585

14685

108110

592

640 tracks

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1. First-Come-First-Served (FCFS)

Easy to program and intrinsically fair.

Ignore position relationships among

pending requests.

Acceptable when the loading on a disk is

light and requests are uniformly

distributed. Not good for medium and

heavy loading.

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2. Shortest-Seek-Time-First (SSTF)

Select the request with minimum seek time from the current head position.

A form of SJF scheduling Better than FCFS in general. It may cause starvation of some

requests.

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2. Shortest-Seek-Time-First (SSTF)


0 14 37 53 65 67 98 122 124 183 199122

302384242

59

236 tracks

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SSTF

It is not optimal Consider the servicing sequence (from

the left): 53, 37, 14, 65, 57, 98, 122, 124, 184.

The total head movement is 208 tracks.

This is 28 tracks less than that of SSTF.

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SSTF not optimal


0 14 37 53 65 67 98 122 124 183 199122

302384242

59

236 tracks(SSTF)

1623512

31242

59

208 tracks

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3. SCAN (Elevator algorithm)

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.

Most disk scheduling strategies actually

implemented based on SCAN

Improve throughput and mean response time.

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0 14 37 53 65 67 98 122 124 183 199162314652

31242

59

236 tracks

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Eliminate much of discrimination in SSTF much lower variance in response time. The requests at the other end of the

disk wait the longest time. But the upper bound on head

movement for servicing a disk request is just twice the number of disk tracks.

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4. C-SCAN

A variant of SCAN which provides a more uniform wait time than SCAN.

The head moves from one end of the disk to the other end, servicing requests as it goes. When it reaches the other end it immediately returns to the beginning of the disk, without servicing any requests on the return trip .

It has very small variance in response time; that is it maintains a more uniform wait time distribution

among the requests.

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4. Circular-SCAN (C-SCAN)


0 14 37 53 65 67 98 122 124 183 199

122

31242

5916

1991423

382 tracks

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5. C-LOOK

C-LOOK are the practical variants of C-SCAN. The head is only moved as far as the last

request in each direction. When there are no requests in the current

direction the head movement is reversed. C-SCAN always move the head from

one end of the disk to the other.

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5. C-LOOK (Modified C-SCAN)


0 14 37 53 65 67 98 122 124 183 199

122

31242

5916

1991423

322 tracks

169

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Selecting a Disk-Scheduling Algorithm

It is possible to develop an optimal algorithm

but the computation needed may not justify

the savings over SSTF or SCAN scheduling

algorithms.

The SCAN and C-SCAN (or LOOK and C-LOOK)

algorithms are more appropriate for systems

that place a heavy load on the disk.

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Selecting a Disk-Scheduling Algorithm

If the queue seldom has more than one outstanding request then all disk scheduling algorithms are degraded to FCFS and thus are effectively equivalent.

Some disk controller manufacturers have moved disk scheduling algorithms into the hardware itself. The OS sends requests to the controller in FCFS

order and the controller queues them and execute them in some more optimal order.

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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.

To use a disk to hold files, the OS still needs to record its own data structures on the disk. Partition the disk into one or more groups of

cylinders Logical formatting for “making a file system”.

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Swap-Space Management

Swap-space -- Virtual memory uses disk space as an extension of main memory.

Goal: to provide best throughput for virtual-memory system.

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Swap-Space

Main Memory

Disk

Swap spaceFile system

Userprogram

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Swap-Space Use

Systems that implement swapping may use swap

space to hold the entire process image, including the

code and data segments. (Paging systems may

simply store pages that have been pushed out of

main memory.)

Size of swap space: few MB to hundreds of MB

UNIX: multiple swap spaces, usually put on separate

disks. Unix copy entire processes between

contiguous disk regions and memory.

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Swap-Space Location 1. normal file system:

simply a large file within the file system easy to implement, but inefficient due to the

cost of traversing the file-system data structure 2. separated disk partition (more

common): No file system or directory structure is placed on

this space. A separate swap-space storage manager is used

to allocate and de-allocate the block (optimized for speed not storage utilization)

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BSD 4.3 Preallocation: allocates swap space when

process starts; holds text segment (the program) and data segment

kernel uses swap maps to track swap-space use

file system is consulted only once for each text segment;

pages from data segment are read in from the file system, or created, and are written to swap space and paged back in as needed.

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Solaris 2 allocates swap space only when a page is

forced out of physical memory, not when the virtual memory page is first created (modern computer has larger main memory)

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Disk Reliability

Disk failure causes a loss of data and significant downtime while disk is

replaced and data restored.

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Disk striping (interleaving)

A group of disks is treated as one storage unit.

Each data block is divided into several sub-blocks.

Each sub-block is stored on a separate disk. This reduces disk block access time and

can fully utilize disk I/O bandwidth. Performance improvement: All disks

transfer their subblocks in parallel

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RAID

RAID: Redundant Arrays of Inexpensive

Disks It improves performance (especially

price performance ratio) and reliability (with duplication of data).

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RAID Level 1

known as Mirroring or shadowing, makes a duplicate of all data files

onto a second disk. 50% disk space utilization the simplest RAID organization.

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RAID 1 : Mirroring (Shadowing):

c : FtDisk

WRITE

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RAID 3 : Block Interleaved Parity

An extra block of parity data is written to a separate disk.

Example if there are 9 disks in the array then sector 0 of

disks 1 to 8 have their parity computed and stored on disk 9. The operation takes place on bit level for each byte.

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Block Interleaved Parity:

1 0 1 0 1 1 1 1 0

Truth Table of XOR

Disk 1 Disk 2 Parity Disk

Input 1 Input 2 Output

1 1 0

1 0 1

0 1 1

0 0 0

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Block Interleaved Parity

Disk 1

1 0 1 0 ? 1 1 1 0

Truth Table of XOR

Disk 2 Disk 3

Input 1 Input 2 Output

1 1 0

1 0 1

0 1 1

0 0 0

1

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Block Interleaved Parity

If one disk crashes we can re-compute the original data from the other data bits plus the parity.

It has been shown that with a RAID of 100 inexpensive disks and 10 parity disks the mean time to data loss (MTDL) is 90 years.

MTDL of a standard large expensive disk is 2 or 3 years.

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RAID 3

Utilization issue: stripping data across a minimum of two drives while

using a third drive to store each byte’s parity bit. 2/3

disk utilization (3 disks)

Performance issue: During writing, because updating any single data

subblock forces the corresponding parity subblock to be recomputed ad rewritten.

Can manage only one data transfer at a time per array (for example, a single read or write)

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RAID Level 5

Similar to RAID 3; all devices are used for data storage, with parity bit recording distributed across all drives.

RAID 5 provides the best combination of over-all data availability and fault-tolerant protection.

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RAID Info

Read the article about RAID in the course

webpage: SunEXpert, March 1996, Vol.

7, No. 3, “RAID: Wasted Days, Wasted

nights”

Documents

Lecture 8: Secondary-Storage Structure