Operating Systems ECE344 Disks and RAID

Ashvin Goel ECE University of Toronto

Outline

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Disks

o o Disk scheduling algorithms Redundancy in storage systems, RAID  File Systems o o o o o Overview of file system File system design Consistency and crash recovery Sharing files Unix file system 2

Disk Geometry

 Disks have multiple platters o o o Each platter has an head The different heads are attached to a single arm The different heads can access data in parallel  Each platter has multiple concentric tracks o A cylinder consists of the same track across different platters  Each track has multiple sectors o A sector typically has preamble, data and ECC Track Platters Sector Cylinder 3

Disk Access Delays

 Time to access a disk sector is determined by 3 delays o o o Seek time  Time to move head to correct track Rotational delay  Time for disk to rotate to correct sector Transfer time  Time to read/write the bits of sector 4

Disk Performance Trends

 Capacity o 100% per year (2X every year)  Transfer rate (BW) o o o 40% per year (2X every two years) Typically, 50-100 MB/s Sector transfer time is 100-200 microseconds  Seek time and Rotation time o o o 8% per year (1/2 every 10 years) Seek and rotation time are typically 4-8 ms today Disk scheduling aims to minimize these times, especially seek time 5

Disk Performance

 Fastest: No head movement, no rotational latency o => Sequential access  Faster: no head movement, rotational latency o => Access to blocks on the same cylinder is faster  Slow: head movement, rotational latency o o => Access to blocks on different cylinders => The further the cylinders the worse it is

Addressing Disks

 Older disks required OS to specify all parameters for transferring data o E.g., cylinder #, track #, sector #, transfer size  Modern disks are more complicated o Not all sectors are the same size, sectors are remapped, etc.

 Current disks provide a higher-level interface o o o Disk exports its data as a logical array of blocks Disk maps logical blocks to its surface OS code is simpler but disk parameters are hidden 7

Layers of Abstraction

 Program  File system  Device driver  Disk Controller 8

Disk Errors

 Lots of errors possible o o o E.g., latent sector errors, mis-directed writes, etc.

Transient vs. hard errors Some errors can be masked by ECC  Bad sectors o o o o Allocate spare sectors per track Block can be mapped to spare in various ways  In the factory, by the device controller, by OS OS can hide bad sectors by allocating them to a special hidden file Physical backup programs have to careful 9

Disk Scheduling Algorithms

 Aim is to improve disk performance  Two methods o o Reduce seeks and rotation Read several blocks of data  Algorithms o o o First-come, first served (FCFS)  Simple, fair, slow Shortest seek time first (SSF) SCAN (Elevator) 10

Shortest Seek First (SSF)

Initial position Pending requests  Shortest seek first minimizes arm motion  Unlike FCFS, starvation is possible 11

SCAN (Elevator)

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SCAN (Elevator)

 Use a bit to track outward or inward arm direction  Service the next pending request in same direction  When there are no more requests in the current direction, reverse direction  Increases seek compared to SSF but ensures no starvation  A variant is called C-SCAN o o SCAN, but request go in one direction (typewriter) What are its benefits/drawbacks vs. SCAN?

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Modern Disk Scheduling

 Disks know their layout better than the OS  May ignore or undo disk scheduling in OS!

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Redundancy in Storage Systems

 Idea: Use many disks in parallel o Increases storage bandwidth, improves reliability  Redundant Array of Inexpensive Disks (RAID) o A storage system, not a file system  Files are striped across disks o o o Stripes on different disks can be read/written in parallel Bandwidth increases with more disks Better throughput for large requests  Choosing stripe size is important o o Normally between 1KB to 1MB Increase stripe size, as average file size increases 15

RAID Levels 0, 1

 RAID level 0: disk striping o o Distributes data across several disks for speed No redundancy  RAID level 1: mirroring o o o Backup solution Write both, read either 50% utilization 16

RAID Levels 0, 1

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RAID Levels 4, 5

 RAID level 4: block striping, dedicated parity disk o o o Calculate XOR value of stripes across disks Store XOR stripe value on separate disk Uses one extra disk for parity  Utilization: (N-1)/N, N is the number of disks  RAID level 5: striping with distributed parity o o o Similar to 4 but parity information is distributed across all disks Avoids bottleneck for parity disk If a single disk dies, it has to be replaced  Information for that disk is recreated from the other disks  RAID level 6: o o Can recovery from the loss of two disks Allows (single) failure during disk recovery 18

RAID Levels 4, 5

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