Raid 5: High Availability And Fewer Writes Than Reads - IBM Midrange System DS4000 Series Hardware Manual

RAID 5: High availability and fewer writes than reads

RAID 5 (Figure A-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 A-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 (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.
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.
Disk 1 Disk 2
Block 0 Block 1
Block 5 Block 6
Block 10
Block 11
Block 15
Parity 12-15
Appendix A. Overview of IBM System Storage DS5000 RAID types
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
509