On Thu, 3 Jun 2010, Greg Smith wrote:
And it's also quite reasonable for a RAID controller to respond to that "flush the whole cache" call by flushing its cache.
Remember that the RAID controller is presenting itself to the OS as a large disc, and hiding the individual discs from the OS. Why should the OS care what has actually happened to the individual discs' caches, as long as that "flush the whole cache" command guarantees that the data is persistent. Taking the RAID array as a whole, that happens when the data hits the write-back cache.
The only circumstance where you actually need to flush the data to the individual discs is when you need to take that disc away somewhere else and read it on another system. That's quite a rare use case for a RAID array (http://thedailywtf.com/Articles/RAIDing_Disks.aspx notwithstanding).
If the controller had some logic that said "it's OK to not flush the cache when that call comes in if my battery is working fine", that would make this whole problem go away.
The only place this can be properly sorted is the RAID controller. Anywhere else would be crazy.
Matthew -- "To err is human; to really louse things up requires root privileges." -- Alexander Pope, slightly paraphrased -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
