HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

Other considerations when creating performance pools are:
• Performance pools should be created with back-end LUs that are the same size, otherwise
SVSP uses the capacity of the smallest back-end LU for all the virtual disks.
• SVSP does not allow another back-end LU to be added to a stripe set once a stripe set has
been created.
• SVSP does allow adding multiple stripe sets to a storage pool, but SVSP will not rebalance
as an EVA would. Add capacity to the EVA and present additional virtual disks to the SVSP,
because the EVA will rebalance.
Split I/O
Typically, the DPM maps host I/O to physical storage in wire-speed using the DPM's FastPath
hardware mechanism. However, host I/O will occasionally span two back-end LUs. This is called
a split I/O. Split I/Os are handled on the DPM soft path. I/O handled by the soft path does not
have the lowest latency and highest throughput achieved by the DPM fast path. An occasional
split I/O will have an imperceptible impact. With concatenated storage pools, there are very few
split I/O because there are only as many opportunities for split I/O as there are adjacent back-end
virtual disks. The use of stripe sets, however, increases the likelihood of split I/Os.
Host I/O with a size less than 1 MB, and aligned to 1 MB boundaries, will not be split and therefore
will be faster. Many backup solutions use a large I/O size, which result in retries and high latency.
In addition, VMware has a MaxDiskIOSize setting that restricts I/O to the specified maximum size.
This works in SVSP version 3.0.2 or later, but will result in reduced throughput of the storage
system, pushing this solution into the grey area—meaning that performance should be monitored.
The following link explains how to configure VMware for writes to 128 KB, which is the
recommended size for best performance with SVSP:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&
externalId=1003469
26 Storage pools