Apple Xsan 1.x - Manual (Page 14)

14 Chapter 1 Setup and Tuning Guidelines
If, however, the workflow supported by the volume consists mostly of sequential reads
or writes, as is the case for audio or video streaming or capture, you can get better
performance with a larger block size. Try a 64 KB block size in such cases.
Choosing a Storage Pool Stripe Breadth
The Mac OS X (or Mac OS X Server) operating system, which handles file data transfers
for Xsan, performs 1 megabyte (MB) data transfers. As a result, Xsan gets maximum
efficiency from the operating system when it transfers blocks of data that are a multiple
of 1 MB.
At the other end of the transfer, the LUN also works well when it receives 1 MB of data
at a time. So, when data is written to a storage pool, you get the best performance if
1 MB of data is written to each LUN in the storage pool The amount of data Xsan writes
to a LUN is determined by the product of two values you specify when you set up a
volume:
 The volume’s block allocation size (in kilobytes)
 The stripe breadth of the storage pools that make up the volume (in number of
allocation blocks)
transfer size = block size x stripe breadth
For example, the default Xsan block size of 4 KB combines with the default storage
pool stripe breadth of 256 blocks to produce a transfer size of 1 MB. If you increase the
block size to 64 KB, for example, to suit data streaming, set the stripe breadth to 16
blocks, so the product of the two remains 1 MB.
Choosing a Storage Pool Multipath Method
You can increase performance by attaching each client to the SAN using two Fibre
Channel cables. Xsan can take advantage of multiple Fibre Channel connections to a
client to increase transfer rates. The multipath method you specify for a storage pool
determines how Xsan uses more than one Fibre Channel connection between a client
and a storage pool.
If a client has two Fibre Channel connections to the SAN, you can increase transfer
speeds between the client and a storage pool by setting the storage pool’s multipath
method to Rotate.
Multipath method Description
Rotate Xsan alternates transfers among the available client Fibre Channel
connections. In this case, each transfer uses a different connection
than the transfer before it, so it can be started before the
preceding transfer finishes.
Static Each LUN in the storage pool is assigned to a client Fibre Channel
connection when the volume is mounted.