Dell PowerVault MD3000i - Manual (Page 7)

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
December 2008 – Revision A01 Page 7
Physical disk cost is not the only factor that influences the decision on which
RAID level is most appropriate for a given application. The performance of a
chosen RAID level is heavily interdependent on characteristics of the I/O pattern
as transmitted to the storage array from the host(s). With I/O patterns involving
write operations, when an I/O burst exceeds 1/3 of available cache memory in
size, it should be considered a long I/O. Long writes show the performance of a
chosen RAID level better than short writes. Short write operations can be
handled entirely in cache, and the RAID level performance effect is minimized.
As long as the write-burstiness is always lower than the cache to disk offload
rate, a choice in RAID level can be a non-issue.
In general, the following outlines which RAID levels work best in specific
circumstances:
• RAID 5 and RAID 6 works best for sequential, large I/Os (>256KiB)
• RAID 5 or RAID 1/10 for small I/Os ( <32KiB )
• For I/O sizes in between, the RAID level is dictated by other application
characteristics:
o
RAID 5 and RAID 1/10 have similar characteristics for most Read
environments and sequential Writes.
o
RAID 5 and RAID 6 exhibit the worst performance mostly by
random writes.
o
In random I/O applications consisting of more than 10% write
operations, RAID 1/10 provides the best performance.
Table 1 provides a summary of these points for an ideal environment. An ideal
environment consists of aligned stripes or segments reads and writes, as well as
burst I/O where cache memory and RAID Controller Modules are not overly
saturated by I/O operations.
Table 1: I/O Size and Optimal Raid Level
Significantly Random Significantly Sequential
Block Size Read Write Read Write
Small
(<32 KiB )
1/10, 5, 6 1/10 1/10, 5, 6 1/10, 5
Medium
(Between 32
and 256 KiB)
1/10, 5, 6 1/10 1/10, 5, 6 5
Large
(>256 KiB)
1/10, 5, 6 1/10 1/10, 5, 6 5