Following this article about the X25-E SSD and performance when the SSD's write-cache turned off, I made my own observations.
- Adaptec 5805 + 512MB battery-backed cache
- 4x X25-E 64GB (SSDSA2SH064G1GC) in RAID10, 256KB stripe)
With an active MySQL server with InnoDB tables, I selectively set to write-back/write-through the cache of the the controller (arcconf setcache logicaldrive wt/wbb) and drives ((arcconf setcache device wt/wb) and observed iostat over a 15-second interval.
Fig. 1 - iostat -k 15 -x across four different cache settings
Performance clearly suffers with write cache turned off, with a 3x increase in (write) latency.
What's interesting though is that the controller's cache resolves this problem and improves latency by another 3x over the SSD's write-back performance.
So a RAID controller is a good complement to SSDs when data integrity is important, improving performance by 10x in our case. With the associated risk of the controller fubar'ing and wiping the cache of course :)