Hi Craig, On Fri, 12 Sep 2014 11:33:55 +0800, Craig Ringer <cr...@2ndquadrant.com> wrote:
>On 09/11/2014 03:16 PM, George Neuner wrote: >> >> If the driver permits it and you [or your users] can be trusted to >> perform a safe unmount via the OS *before* disconnecting the device, >> then you can enable write caching for the device using the device >> manager. [Note that the device must be connected for it to be visible >> in the device manager.] > >It shouldn't be living dangerously, actually. > >While I haven't tested it myself, writeback caching on the external >drive should be safe so long as it continues to honour explicit disk >flush requests. > >That's why we have the WAL and do periodic checkpoints. If you yank the >drive mid-write you'll lose uncommitted transactions and might have >slower startup next time around, but it should otherwise not be overly >problematic. For the most part you're correct, but recall that WAL itself can be made asynchronous [see fsync() and synchronous_commit() settings] and the periodic OS sync also may be disabled - which doesn't affect WAL handling but may(?) affect the background writer. Even having synchronous WAL the most recent transactions can be lost if the log device fails *during* a write. That's why, if we use external devices at all, we tend to use closely coupled devices - disk array, wired SAN, etc. - that aren't very likely to be physically disconnected. And uninterruptible power all around 8-) A portable device can be reasonably safe if treated properly, but it never will be quite as safe as an internal device. George -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
