Apple Xsan 1.x - Manual (Page 12)

12 Chapter 1 Setup and Tuning Guidelines
Although some large write requests might benefit from caching, often they do not.
By placing a volume’s metadata storage pool on a controller separate from the data
storage pools, you can configure the metadata controller to use caching and the data
controller to run without caching.
When the file system is relying on caching in this way, you must guarantee that data in
the cache isn’t lost before it is actually written to disk. Data that has been written to
disk is safe if the power fails, but data in a cache is not. So, to be sure that a power
failure can’t cause the loss of cached data, protect your Xserve RAID systems with
controller backup batteries or an uninterruptable power supply (UPS).
Important: If you enable Controller Write Cache on an Xserve RAID system, be sure that
the system includes controller backup batteries and, preferably, is connected to a UPS.
To enable Xserve RAID write cache:
m Open the RAID Admin application, select the RAID system, and click Settings. Then click
Performance and enable Write Cache for each controller.
Set Read Prefetch to 8 Stripes
Read prefetch is a technique that improves file system read performance in cases
where data is being read sequentially, as in the case of audio or video streaming, for
example. When read prefetch is enabled, the controller assumes that a read request for
a particular block of data will be followed by requests for subsequent, adjacent data
blocks. To prepare for these requests, the controller reads not only the requested data,
but also the following data, and stores it in cache memory. Then, if the data is actually
requested, it is retrieved from the fast cache instead of from the slow disk drives.
Read prefetch is always enabled on Xserve RAID systems, though you can adjust the
amount of data that is read. If you’re using other RAID systems, check the
documentation to find out how to enable read prefetch.
To adjust the Xserve RAID read prefetch size:
m Open the RAID Admin application, select the RAID system, and click Settings. Then click
Performance and select a Read Prefetch size for each controller.
The default of 8 stripes is best for most applications.
Estimating Base Xserve RAID Throughput
To estimate how many Xserve RAID systems you need to support specific throughput
requirements, you can assume that one Xserve RAID with 14 drives set up as two RAID
5 arrays can handle a minimum of 160 MB of data per second (80 MB/s per RAID
controller). This value is applicable to video streaming applications; other applications
might achieve higher data rates.
Overall performance is also affected by SAN latency; see “About SAN Write Latency” on
page 15.