On 2/27/07, Shane Ambler <pgsql@xxxxxxxxxx> wrote:
Jeff Davis wrote: >> Sorry for for not being familar with storage techonologies... Does >> "battery" here mean battery in the common sense of the word - some >> kind of independent power supply? Shouldn't the disk itself be backed >> by a battery? As should the entire storage subsystem? >> > > Yes, a battery that can hold power to keep data alive in the write cache > in case of power failure, etc., for a long enough time to recover and > commit the data to disk. Just to expand a bit - the battery backup options are available on some raid cards - that is where you would be looking for it. I don't know of any hard drives that have it built in. Of cause another reason to have a UPS for the server - keep it running long enough after the clients have gone down so that it can ensure everything is on disk and shuts down properly. > So, a write cache is OK (even for pg_xlog) if it is durable (i.e. on > permanent storage or backed by enough power to make sure it gets there). > However, if PostgreSQL has no way to know whether a write is durable or > not, it can't guarantee the data is safe. > > The reason this becomes an issue is that many consumer-grade disks have > write cache enabled by default and no way to make sure the cached data > actually gets written. So, essentially, these disks "lie" and say they > wrote the data, when in reality, it's in volatile memory. It's > recommended that you disable write cache on such a device. From all that I have heard this is another advantage of SCSI disks - they honor these settings as you would expect - many IDE/SATA disks often say "sure I'll disable the cache" but continue to use it or don't retain the setting after restart.
As far as I know, SCSI drives also have "write cache" which is turned off by default, but can be turned on (e.g. with the sdparm utility on Linux). The reason I am so much interested in how write cache is typically used (on or off) is that I recently ran our benchmarks on a machine with SCSI disks and those benchmarks with high commit ratio suffered significantly compared to our previous results "traditionally" obtained on machines with IDE drives. I wonder if running a machine on a UPS + 1 hot standby internal PS is equivalent, in terms of data integrity, to using battery backed write cache. Instinctively, I'd think that UPS + 1 hot standby internal PS is better, since this setup also provides for the disk to actually write out the content of the cache -- as you pointed out. Thanks Peter
-- Shane Ambler pgSQL@xxxxxxxxxx Get Sheeky @ http://Sheeky.Biz
