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.
--
Shane Ambler
pgSQL@xxxxxxxxxx
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