Apple Xsan 1.x - Manual (Page 11)

Chapter 1 Setup and Tuning Guidelines 11
Adjusting Xserve RAID Fibre Channel Settings
There are several Xserve RAID settings that can affect the Fibre Channel performance of
the device and the SAN as a whole.
Fibre Channel Speed
Be sure the Fibre Channel connection is set to operate at 2 GB/s.
Fibre Channel Topology
To add an Xserve RAID system to a Fibre Channel fabric, set the topology to Automatic.
Disable Hard Loop ID
Don’t enable hard loop IDs for Xserve RAID systems in a Fibre Channel fabric.
To adjust Xserve RAID Fibre Channel settings:
m Open RAID Admin, choose a system, click Settings, and enter the management
password for the system. Then click Fibre Channel.
Adjusting Xserve RAID Performance Settings
Xserve RAID performance settings, affecting parameters such as drive caching,
controller caching, and read prefetching, can have a significant effect on Xsan volume
performance. Follow these guidelines.
Enable Drive Cache
In addition to the caching performed by the Xserve RAID controller, each drive in an
array can perform its own caching at the drive level to improve performance.
Important: If you enable drive cache for an Xserve RAID set, be sure that the system is
connected to a UPS. Otherwise, you could lose cached data if the power fails.
To enable drive cache for an Xserve RAID array:
m Open the RAID Admin application, select the RAID system, and click Settings. Then click
Performance and enable Drive Cache for the array.
Enable Controller Write Cache
Without RAID controller write caching, a request to write data to the associated LUN is
not considered finished until the data has been completely written to the physical disks
that make up the array. Only then can the next write request be processed. (This is
sometimes called “write-through caching.”)
When the RAID controller write cache is enabled, a request to write data is considered
finished as soon as 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.
Always be sure to enable write caching on controllers that support metadata storage
pools.