HP StorageWorks SAN Virtualization Services Platform Administrator Guide (5697-0934, May 2011)

required capacity) in order to save time (at least for PiT expansions time is a critical factor).
When additional virtual disks are configured, a back-end volume is selected at random, and
the algorithm seeks to find a contiguous free space. This applies to the temporary volumes
used to support PiTs, snapshots, thin provisioning, and so on, too.
• One tradeoff of this simple general purpose pool construction approach is that it uses back-end
volumes and back-end paths in quantities that can run out before achieving the maximum
capacity or maximum number of arrays. The maximum number of back-end volumes supported
in SVSP v3 is 1024 (512 in versions before v3). The maximum number of back-end paths
supported to each back-end volume is eight. The maximum number of back-end paths per
DPM is 4K (these correspond to a data structure called physical storage containers (PSCs).
Building storage pools using stripe sets
The capacity-optimized pools described above are a good starting point, but it may not be the
best choice for all situations. One issue is that the use of the multiple paths to the back-end volumes
is “hit or miss.” It is usually much better than a construction that uses a small number of paths, but
the use of the multiple paths is not deterministic. Storage pools that are built using stripe sets spread
the I/O for a single virtual disk across the many paths and back-end volumes used to build these
performance pools. As a result, a front-end virtual disk that is created from a pool built with stripe
sets will be sequentially spread across all the members of the stripe set in 1-MB chunks.
Build a stripe set using the same guidelines as those for building a general purpose or
capacity-based storage pool. That is, the volumes should be of the same RAID type, with similar
capacity and performance characteristics. The size of a stripe set is based on the size of its smallest
member times the number of members. This means that unless all members are of exactly the same
size there will be capacity that is not accessible. The stripe sets should be constructed of at least
as many volumes as there are paths from a single DPM to the array. One or more stripe sets are
used to create the pool.
Stripe sets can both improve performance or degrade performance so it is important to understand
the following guidelines:
• Sequential I/O or transactional I/O—If the I/O stream is largely sequential, and the array is
able to detect a sequential stream, it may make more sense to use a pool over a stripe set.
Additionally, if the I/O size is greater than one megabyte, striping it would create two requests.
If the I/O stream is largely transactional, a stripe set will probably perform better.
• SVSP does not allow another back-end LU to be added to a stripe set.
• SVSP does allow the addition of a stripe set to a pool, but SVSP will not rebalance. A best
practice in this area is to add capacity to the HP EVA and present additional volumes to the
SVSP, because the HP EVA will rebalance.
Storage pool size considerations
When comparing small pools to large pools, the large pools have an advantage. Because there
are fewer, they are easier to manage, and since the pool free space in the same pool is used for
snapshots, asynchronous mirroring, and thin provisioning, there is a less likelihood of stranded
capacity. Small pools, however, may allow the administrator to better partition the storage for
various user groups, or to have a pool per back-end array to ease troubleshooting.
Using thinly provisioned virtual disks
In general, a thin volume has similar performance characteristics to those of a regular volume;
however, when additional capacity is required by a thin volume, additional time is needed to
complete the write. This may be observed after the creation of a thinly provisioned virtual disk
when random writes may trigger many expansions, and is less likely to occur after a volume has
been used for a while. To avoid this first write penalty, pre-write a significant portion of the volume
and then delete the data. SVSP allows for an initial allocation of up to (the smaller of) 10% of the
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