On 3/8/07, Magnus Hagander <magnus@hagander.net> wrote:
> On Thu, Mar 08, 2007 at 06:24:35AM -0000, James Mansion wrote:
> >
> > In the long run, we are going to have to seriously rethink pg's use
> > of WAL as the way we implement MVCC as it becomes more and more of a
> > performance bottleneck.
> > We have WAL because Stonebreaker made an assumption about the future
> > dominance of optical media that has turned out to be false.
> > ...and it's been one of pg's big issues every since.
>
> Uh. pg didn't *have* WAL back when Stonebreaker was working on it. It
> was added in PostgreSQL 7.1, by Vadim. And it significantly increased
> performance at the time, since we no longer had to sync the datafiles
> after every transaction commit.
> (We also didn't have MVCC back in the Stonebreaker days - it was added
> in 6.5)
Exactly, and WAL services other purposes than minimizing the penalty
from writing to high latency media. WAL underlies PITR for example.
Near-zero latency media is coming, eventually...and I don't think the
issue is reliability (catastrophic failure is extremely unlikely) but
cost. I think the poor write performance is not an issue because you
can assemble drives in a giant raid 0 (or even 00 or 000) which will
blow away disk based raid 10 systems at virtually everything.
Solid State Drives consume less power (a big deal in server farms) and
the storage density and life-span will continue to improve. I give it
five years (maybe less) before you start to see SSD penetration in a
big way. It will simply become cheaper to build a box with SSD than
without since you won't need to buy as much RAM, draws less power, and
is much more reliable.
Disk drives will displace tape as low speed archival storage but will
probably live on in super high storage enterprise environments.
my 0.02$, as usual,
merlin