Adjust RAID system performance settings
To get the best performance and reliability from your RAID systems, install the latest firmware.
RAID system performance settings, which affect parameters such as drive caching, RAID controller caching, and read prefetching, can have a significant effect on Xsan volume performance. Follow these guidelines.
Enable drive caching
In addition to the caching performed by the RAID controller, each drive in an array can perform caching at the drive level to improve performance.
Enable RAID controller write caching
Without RAID controller write caching, a request to write data to the associated LUN isn’t considered finished until the data is written to the physical disks that make up the array. Only then can the next write request be processed. (This is also known as write-through caching.)
WARNING: If you enable drive caching for a RAID set, make sure the system is connected to an uninterruptible power supply (UPS). Otherwise, you could lose cached data if the power fails.
When RAID controller write caching is enabled, a request to write data is considered finished when the data is in the cache. This is also known as 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 isn’t. 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.
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.