CSCI 3431: OPERATING SYSTEMS

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

Magnetic Disk Terminology

Transfer Rate: Rate at which data flows from the drive 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

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 if it fulfills a role e.g., magnetic tapes for second-level

backups of financial data (still in use on mainframe systems – cost per bit very low compared to disks)

Head Scheduling Concerns

Synchronicity: Must requests be processed in order?

Average Wait Time: How long is a process typically blocked?

Maximum Wait Time: How long could a process be 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

cylinder order 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 algorithm based on data properties (like sorting functions in libraries)

Disk Management

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

Blocks typically 512K, but 256 and 1024 are sometimes possible

OS uses "Clusters" which are larger than blocks 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

disk controller Spare sectors exist Can cause problems with head

scheduling Controllers try to generate

replacements on the 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 used separately Often provided as a "unit" with its

own independent controller

Mirroring

Simplest form of redundancy Both disks have to fail to lose data About the most effective method for

providing 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 to

power-failure

Striping

Various Levels Block-level (most common) Byte-level Bit-level

RAID Levels

0: Block-striping, no redundancy1: Complete mirroring2: Byte-striping, parity disks, ECC for single bit errors3: Bit-interleaved parity (uses sector parity checks to

reduce overhead of RAID 2)4: Block-interleaved parity (RAID 0 with parity blocks)5: Distributed block-interleaved parity: Same as 4 but

with parity and data intermixed6: P+Q Scheme: Same as 5, but with improved ECC

such as Reed-Solomon coding (tolerates dual disk failures)

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 due to mirroring Simple to implement and maintain

Selecting a RAID Level

Rebuild Time: How long does it take to react to a failure?

Need for reliability? Cost effectiveness (mirroring costs more per

bit)? Performance concerns Type of protection – RAID protects against

hardware 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) – Various technologies 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 (including latency)

Sustained Bandwidth: Rate of flow Average Latency: Time from request until transfer

begins Reliability (Redundancy) – number and kind of

failures from which recovery can occur Cost – price per bit Lifespan – Will data be required in 20 years? Access Frequency – How often will data be

accessed?

To Do:

Work on Assignment 2 (Due next week)

Complete Lab 7 (optional lab) Read Chapter 12 (pgs 505-545; this

lecture)