Dell PowerVault MD3000i Dell PowerVault MD3000/MD3000i Array Tuning Best Pract - Page 7

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices, Significantly Random, Significantly - cost

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Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices 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 (

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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