Redundant Array of Inexpensive (Independent) Disks
– Use multiple smaller disks (c.f. one large disk)
– Parallelism improves performance
– Plus extra disk(s) for redundant data storage
Provides fault tolerant storage system
– Especially if failed disks can be “hot swapped”
RAID 0
– No redundancy (“AID”?)
Just stripe data over multiple disks
– But it does improve performance
“allocation of logically sequential data blocks to separate disks to allow higher performance than a single disk can deliver”
RAID
I/O Errors 1
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RAID 1: Mirroring
– N + N disks, replicate data
Write data to both data disk and mirror disk On disk failure, read from mirror
RAID 2: Error correcting code (ECC)
– N + E disks (e.g., 10 + 4)
– Split data at bit level across N disks
– Generate E-bit ECC
– Too complex, not used in practice (for disks, but…)
RAID 1 & 2
I/O Errors 2
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N + 1 disks
– Data striped across N disks at byte level
– Redundant disk stores parity
– Read access
Read all disks
– Write access
Generate new parity and update all disks
– On failure
Use parity to reconstruct missing data
Not widely used
RAID 3: Bit-Interleaved Parity
I/O Errors 3
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N + 1 disks
– Data striped across N disks at block level
– Redundant disk stores parity for a group of blocks
– Read access
Read only the disk holding the required block
– Write access
Just read disk containing modified block, and parity disk Calculate new parity, update data disk and parity disk
– On failure
Use parity to reconstruct missing data
Not widely used
RAID 4: Block-Interleaved Parity
I/O Errors 4
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RAID 3 vs RAID 4
I/O Errors 5
CS@VT Computer Organization II ©2005-2013 McQuain
N + 1 disks
– Like RAID 4, but parity blocks distributed across disks
Avoids parity disk being a bottleneck Widely used
RAID 5: Distributed Parity
I/O Errors 6
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N + 2 disks
– Like RAID 5, but two lots of parity
– Greater fault tolerance through more redundancy
RAID 6: P + Q Redundancy
I/O Errors 7
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RAID can improve performance and availability – High availability requires hot swapping
Assumes independent disk failures
– Too bad if the building burns down!
See “Hard Disk Performance, Quality and Reliability” – http://www.pcguide.com/ref/hdd/perf/index.htm
RAID Summary
I/O Errors 8
CS@VT Computer Organization II ©2005-2013 McQuain