IBM Midrange System DS4000 Series Hardware Manual page 124

Performance is very good for large amounts of data, but poor for small requests because
every drive is always involved, and there can be no overlapped or independent operations. It
is well-suited for large data objects such as CAD/CAM or image files, or applications requiring
sequential access to large data files. Select RAID 3 for applications that process large blocks
of data. It provides redundancy without the high impact incurred by mirroring in RAID 1.
RAID 5: High availability and fewer writes than reads
RAID 5 (Figure 4-3) stripes data and parity across all drives in the array. RAID 5 offers both
data protection and increased throughput. When you assign RAID 5 to an array, the capacity
of the array is reduced by the capacity of one drive (for data-parity storage). RAID 5 gives you
higher capacity than RAID 1, but RAID level 1 offers better performance.
Figure 4-3 RAID 5
RAID 5 is best used in environments requiring high availability and fewer writes than reads.
RAID 5 is good for multi-user environments, such as database or file system storage, where
typical I/O size is small, and there is a high proportion of read activity. Applications with a low
read percentage (random write-intensive) do not perform as well on RAID 5 logical drives
because of the way a controller writes data and redundancy data to the drives in a RAID 5
array. If there is a low percentage of read activity relative to write activity, consider changing
the RAID level of an array for faster performance; however, if the write activity consists of
sequential, large block I/O, then RAID 5 is the best option.
Use write caching on RAID 5 arrays, because RAID 5 writes will not be completed until at
least two reads and two writes have occurred. The response time of writes will be improved
through the use of write cache (be sure it is battery-backed up). RAID 5 arrays with caching
can give as good as performance as any other RAID level, and with some workloads, the
striping effect gives better performance than RAID 1. For better cache usage, you can double
the cache memory space by disabling the write cache mirror, so the controllers will not have a
copy of the other cache data. Do that action only when data can be rewritten if it is lost,
because if there is a controller failure, the cache data is lost.
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IBM Midrange System Storage Hardware Guide
Disk 1 Disk 2
Block 0 Block 1
Block 5 Block 6
Block 10
Block 11
Block 15
Parity 12-15
Logical Drive
Block 0
Block 1
Block 2
Host View
Block 3
Block 4
Block 5
etc.
Disk 3 Disk 4
Block 2 Block 3
Block 7 Parity 4-7
Parity 8-11
Block 8
Block 12
Block 13
RAIDset
Disk 5
Parity 0-3
Block 4
Block 9
Block 14