CSCI 3431: Operating Systems

Chapter 12 – Mass Storage Structures (Pgs 505-545 )

Magnetic Disk Terminology

Transfer Rate: Rate at which data flows from thedrive to main memory

Positioning Time

(time before IO can occur)

Seek Time

(time to move the head)

Rotational Latency

(time for correct sector to come under head)

Disk Construction

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Block:

512 Bytes (typical)

Disk Bus Formats

Always changing and improving

SCSI: Small computer systems interface

ATA: Advance technology attachment

SATA: Serial ATA

EIDE: Enhanced integrated drive electronics

USB: Universal serial bus

FC: Fibre channel

The Future of Disks

Surviving for the near future

Slower than persistent RAM

Cheaper than persistent RAM

Older technology tends to remain available ifit fulfills a role

e.g., magnetic tapes for second-level backupsof financial data (still in use on mainframesystems – cost per bit very low compared todisks)

Head Scheduling Concerns

Synchronicity: Must requests be processed inorder?

Average Wait Time: How long is a processtypically blocked?

Maximum Wait Time: How long could a processbe blocked (Important in RTOS)

Priority: Is all IO equally important?

** Almost identical to process scheduling **

Scheduling Algorithms

First Come, First Served (FCFS)

Fair but not most efficient

Shortest seek time first (SSTF)

Better, but far from optimal

SCAN (Elevator)

Moves back and forth, servicing requests in cylinderorder

C-SCAN (Circular)

Same as SCAN but don't service on return trip

LOOK (and also C-LOOK)

Same as SCAN but only go/return as far as needed

Choosing One

Often don't have to now

being built into the disk controller

SSTF or LOOK are most common

Simple, decent performance

Sophisticated systems change algorithmbased on data properties (like sortingfunctions in libraries)

Disk Management

Much of the low-level management (blocks,ECC, bad-sector replacement) is done by thedisk controller

Blocks typically 512K, but 256 and 1024 aresometimes possible

OS uses "Clusters" which are larger thanblocks and which function as virtual blocks

Increases efficiency, better supports large files,reduces overhead, maps to page size

Bad Blocks

Cheap disks don't do much

Newer/High-end disks handle it via diskcontroller

Spare sectors exist

Can cause problems with head scheduling

Controllers try to generate replacements onthe same cylinder for this reason

Small errors often repairable using ECC

RAID

Redundant Array of Independent Disks

Improved reliability

Higher data transfer rate

Less storage (due to redundancy) than if usedseparately

Often provided as a "unit" with its ownindependent controller

Mirroring

Simplest form of redundancy

Both disks have to fail to lose data

About the most effective method forproviding redundancy

Highly expensive with respect to resources

"Independent Failure" is not fully realistic

Bad batches, aging, fire/flood, etc.

NOTHING prevents some data loss due topower-failure

Striping

Various Levels

Block-level (most common)

Byte-level

Bit-level

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RAID Levels

0: Block-striping, no redundancy

1: Complete mirroring

2: Byte-striping, parity disks, ECC for single bit errors

3: Bit-interleaved parity (uses sector parity checks toreduce overhead of RAID 2)

4: Block-interleaved parity (RAID 0 with parity blocks)

5: Distributed block-interleaved parity: Same as 4 butwith parity and data intermixed

6: P+Q Scheme: Same as 5, but with improved ECCsuch as Reed-Solomon coding (tolerates dual diskfailures)

RAID 0 + 1

RAID 0: Striping, improves performance

RAID 1: Mirroring, improves reliability

Put together they are highly effective

Only half the storage capacity is available dueto mirroring

Simple to implement and maintain

Selecting a RAID Level

Rebuild Time: How long does it take to react to afailure?

Need for reliability?

Cost effectiveness (mirroring costs more perbit)?

Performance concerns

Type of protection – RAID protects againsthardware failures, not against software failures

Properties of file systems – stability, volumes,sizes

Other Storage

NVRAM – As a solid state disk or as a write-back cache

Optical Disks (CD/DVD) – Varioustechnologies for CD-ROM, CD-R ("WORM"),CD-RW

Magnetic Tapes: Cartridges (kind of like VHS),slow but very economical

Storage Performance Issues

Speed -- Bandwidth & Latency

Effective Bandwidth: Overall transfer rate (includinglatency)

Sustained Bandwidth: Rate of flow

Average Latency: Time from request until transferbegins

Reliability (Redundancy) – number and kind offailures from which recovery can occur

Cost – price per bit

Lifespan – Will data be required in 20 years?

Access Frequency – How often will data beaccessed?

To Do:

Work on Assignment 2 (Due next week)

Complete Lab 7 (optional lab)

Read Chapter 12 (pgs 505-545; this lecture)