Now let’s see if it makes sense to buy the battery, i.e. to enable Write Back mode. The number in each table cell is the ratio of the array speed in WriteBack mode to its speed in WriteThrough mode. A bigger number is indicative of a higher value of the WriteBack caching policy (and of the battery, too!)
Write-caching influences the performance of all the arrays more when there’s a higher percentage of writes and a longer request queue. It helps the RAID0 and RAID10 arrays perform faster, but the RAID5 is slower under high loads when write-caching is enabled.
We built diagrams with four disk arrays in WriteThrough and WriteBack modes for 1, 16 and 256 queue depths.
Quite expectedly, the performance of the RAID0 array degenerates in all modes, expect Random Read, when write-caching is disabled. The maximum performance hit amounts to 478%, at linear load.