Apple Xsan 2 - Manual (Page 55)

Chapter 3 Plan a SAN 55
When RAID controller write caching is enabled, a request to write data is considered
finished when the data is in the cache. This is sometimes called write-back caching.
Write requests are processed more quickly because the file system only needs to write
to the fast cache memory and doesn’t need to wait for the slower disk drives.
Be sure to enable write caching on RAID controllers that support metadata storage pools.
Although some large write requests might benefit from caching, often they don’t. By
placing a volume’s metadata storage pool on a RAID controller separate from the data
storage pools, you can enable caching on the RAID controller used for metadata and
disable caching on the RAID controller used for data.
When the file system is relying on caching in this way, you must guarantee that data
in the cache isn’t lost before it’s written to disk. Data written to disk is safe if the power
fails, but data in a cache is not. To be sure that a power failure can’t cause the loss of
cached data, protect your RAID systems with RAID controller backup batteries or a UPS.
WARNING: If you enable controller write caching on a RAID system, make sure the
system includes controller backup batteries and, preferably, is connected to a UPS.
Enable read prefetching
Read prefetching is a technique that improves file system read performance when
data is being read sequentially, as in the case of audio or video streaming, for example.
When read prefetching is enabled, the RAID controller assumes that a read request for
a block of data will be followed by requests for adjacent data blocks. To prepare for
these requests, the RAID controller reads the requested data and the following data,
and stores it in cache memory. Then, if the data is requested, it’s retrieved from the fast
cache instead of from the slower disk drives.