Configuring RAID for Optimal Performance

Configuring RAID for Optimal Performance Impact of RAID Settings on Performance
6
4. Impact of RAID Settings on Performance
4.1 Write Policy
The Write Policy can have a very big impact on write performance. There are two modes
available – Write Back and Write Thru.
4.1.1 Write Back Mode
This mode provides better performance in most cases. In Write-Back mode, the RAID controller
acknowledges write I/O requests immediately after the data loads into the controller cache. The
application can continue working without waiting for the data to be physically written to the hard
drives.
If a power loss occurs in write-back mode, there is a risk of losing data in the RAID cache. The
data loss may be fatal and may require restoring data from a backup device. It is critical to have
protection against power failures. Using a UPS with redundant system power supplies is highly
recommended. RAID Backup Battery Unit can provide additional protection.
4.1.2 Write Thru Mode
This mode does not utilize the RAID cache for accelerating write I/O requests. In most cases it
will be slower than Write-Back mode. However, Write Thru mode allows achieving the highest
sequential write bandwidth with RAID 0 or RAID 10.
4.2 Disk Cache Policy
Disk Cache Policy determines whether the hard-drive write cache is enabled or disabled. When
Write Policy is set to Write Thru mode, Disk Cache Policy can have very big impact on write
performance. When Write Policy is set to Write Back mode, impact of Disk Cache Policy is
much smaller and in many cases negligible.
When Disk Cache Policy is enabled, there is a risk of losing data in the hard drive cache if a
power failure occurs. The data loss may be fatal and may require restoring the data from a
backup device. It is critical to have protection against power failures. Using a UPS with
redundant system power supplies is highly recommended. RAID Backup Battery Unit can
provide additional protection.
Note: A RAID Backup Battery Unit does not protect the hard drive cache.