Re: pg_xlog on separate drive

Thanks for the replies guys. I think I may be ok in my case though because
I'll be importing data as a single daily batch from raw data. I'll be taking
nightly backups, and in the event of a crash, I can simply restore from a
recent backup and then reimport the raw data. I can now see why losing
pg_xlog would be a big problem if I were inserting and updating data
continuously throughout the day though.

Thanks,
Travis

On 12/4/06, Markus Schaber <schabi(at)logix-tt(dot)com> wrote:
>
> Hi, Travis,
>
> Travis Whitton wrote:
> > Hey guys, sorry if this is slightly OT for this list, but I figure it's
> > a simple question. If I'm storing pg_xlog on a second non-redundant
> > drive using the symlink method and the journal drive were to crash, how
> > difficult is recovery? Will Postgresql simply be able to reinitialize
> > the journal on a new drive and carry on, or is there more to it than
> > that? I realize any pending transactions would be lost, but that's not a
> > huge concern for me because everything I'm importing comes from raw
> data.
>
> The problem is that you risk inconsistency at data and structural level.
>
> When the server crashes, it might happen that some pages in the data
> files are written only partially (because most disks have a much smaller
> blocksize than the PostgreSQL page size (which is 8k by default)).
>
> Now, when the server cannot reply the WAL log, those half-written pages
> will not be repaired, and your data may be inconsistent at a very low
> sematic level (duplicate rows, missing rows, broken rows, backend
> crashes etc.) with no way to repair.
>
> HTH,
> Markus
> --
> Markus Schaber | Logical Tracking&Tracing International AG
> Dipl. Inf. | Software Development GIS
>
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