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RAID Benefits and Tradeoffs
2-13
RAID Types and Tradeoffs
Table 2-1 Performance, Availability, and Cost of RAID Types (Individual Unit = 1.0)
Performance
RAID 5, with individual access, provides high read throughput for
small requests (blocks of 2 to 8 Kbytes) by allowing simultaneous
reads from each disk in the group. RAID 5 write throughput is
limited by the need to perform four I/Os per request (I/Os to read
and write data and parity information). However, write caching
improves RAID 5 write performance.
RAID 3, with parallel access, provides high throughput for
sequential, large block-size requests (blocks of more than 64 Kbytes).
With RAID 3, the system accesses all five disks in each request but
need not read data and parity before writing – advantageous for large
requests but not for small ones. RAID 3 employs SP memory without
caching, which means you do not need the second SP and BBU that
caching requires.
Generally, the performance of a RAID 3 Group increases as the size of
the I/O request increases. Read performance increases rapidly with
read requests up to 1Mbyte. Write performance increases greatly for
sequential write requests that are greater than 256 Kbytes. For
applications issuing very large I/O requests, a RAID 3 LUN provides
significantly better write performance than a RAID 5 LUN.
Disk configuration
Relative read
performance
without cache
Relative write
performance
without cache
Relative
cost per
Mbyte
RAID 5 Group with
fivedisks
Up to 5 with five disks
(for small I/O requests, 2
to 8 Kbytes)
Up to 1.25 with five disks
(for small I/O requests, 2 to
8 Kbytes)
1.25
RAID 3 Group with
fivedisks
Up to 4 (for large I/O
requests)
Up to 4 (for large I/O
requests)
1.25
RAID 1 mirrored pair Up to 2 Up to 1 2
RAID 1/0 Group with
10 disks
Up to 10 Up to 5
Individual unit 1 1 1
Notes: These performance numbers are not based on storage-system caching. With caching,
the performance numbers for RAID 5 writes improve significantly.
Performance multipliers vary with load on server and storage system.