scott.marlowe wrote:
> On Fri, 3 Oct 2003, scott.marlowe wrote:
>
> > On Fri, 3 Oct 2003, Tom Lane wrote:
> > >
> > > It'd be interesting to think about whether a write-caching IDE drive
> > > could safely be used for data storage, if WAL is elsewhere.
> >
> > Well, I just so happen to have a machine with two drives in it. I'll get
> > back to you on that.
>
> Ok, I just tested it. I put pg_xlog and pg_clog on a drive that was set
> to write cache disabled, and left the data on a drive where caching was
> enabled. The tps on a pgbench -c 5 -t 500 on the single drive was 45 to
> 55. With the pg_[xc]log moved to another drive and all, I got up to 108
> tps. About double performance, as you'd expect. I didn't test the data
> drive with write caching disabled, but my guess is it wouldn't be any
> slower since pgsql doesn't wait on the rest.
>
> I pulled the plug three times, and all three times the database came up in
> recovery mode and sucessfully recovered. I didn't bother testing to see
> if write caching would corrupt it as I'm pretty sure it would, it
> certainly did when everything was on one drive.
You would have had to pull the plug between the time the system did a
checkpoint (and wrote to the write cache), and before it flushed the
write cache to disk --- no idea how you would find that window, but my
guess is that if you pulled the plug right after the checkpoint
completed, the WAL recovery would fail.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
