Thursday, December 20, 2012

Performance benefits of enabling write caching and using a RAID controller that supports a battery backed write cache


The white line represents the Avg. Disk Write Queue Length

When the white line is at the high point on the chart:
- The RAID controller cache was configured to 100% read, 0% write
- The queue length is 7, meaning there are 7 write operations waiting in queue
- System responsiveness is choppy, with users reporting long delays

When the white line is at the low point on the chart:
- The RAID controller cache was configured to 25% read, 75% write
- The queue length is 0, meaning there are 0 write operations waiting in queue
- System responsiveness is "normal," with users reporting no issues

In this example
- The server is using an HP P400 RAID controller
- 4 physical HDDs in a RAID 1+0 configuration (stripe sets that are mirrored)

Notes
- Data in a controller cache or HDD cache will be lost during a power outage, so a battery backed cache is recommended
- HP disables the "on the HDD" cache for HDDs participating in a RAID array by default. In my tests with the "on the HDD" cache enabled, there was a slight performance loss