Intel RAID Controllers - Best Practices white paper

Intel® RAID Controllers Best Practices White Paper
Revision 1.0
7
The following table provides a quick reference for RAID settings. The information is simplified
and may not be accurate with some applications or tests. For detailed performance tuning
information please refer to the Intel
®
RAID Controller Performance Optimization whitepaper,
available at http://support.intel.com/support/motherboards/server/
.
Read Cache Policy Direct I/O
Read Ahead Policy Adaptive Read Ahead
Write Cache Policy Write Back*
* A RAID controller battery should be used whenever virtual drive write-back cache is enabled and data is mission
critical.
4.2 Hard Disk Cache
Disk drive cache can be enabled in the virtual drive properties page of the RAID configuration
utility. There is a risk of data loss using a hard drive cache; an overview is provided below.
Hard disk drive cache is located within the logic of the hard drive. Cache provides enhanced
performance for sequential read access by retrieving adjacent data on the drive into the data
buffer in case the host computer requests it. This process allows the data to be directly
transferred from the drive’s memory when it is requested rather than waiting for a disk access,
which results in lower latency. Enabling the hard drive cache can also improve write
performance by providing additional memory space for queued data. Write data can be queued
in the disk cache and reported as written even though the data will not move from memory to
the disk until disk access is available. This reduces the delay during disk I/O operations.
There is inherent risk in holding data in the drive cache when a write has been acknowledged
as complete but hasn’t been written to the disk. If the drive loses power the data in the cache
will be lost before it is written to the disk that can cause a “hole” in a data file, which makes the
file unusable. Using a UPS will mitigate this risk but not eliminate it.
Note: A soft or hard reset (<Ctrl> + <Alt> + <Del> or the reset button) does not affect the
completion of a disk write operation because the disk cache will be flushed as long as drive
power is maintained.