Re: PostgreSQL Portable

Hey,
I had many external hard drive crash (savage unplug, power off, pc forced
restart).
The server on the virtual machine was never hurt, nor the data.

Cheers,
Rémi-C

2014-09-12 15:34 GMT+02:00 George Neuner <gneuner2(at)comcast(dot)net>:

> Hi Craig,
>
> On Fri, 12 Sep 2014 11:33:55 +0800, Craig Ringer
> <craig(at)2ndquadrant(dot)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(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general
>