Re: Page Checksums + Double Writes

On Thu, Dec 22, 2011 at 1:50 PM, Jignesh Shah <jkshah(at)gmail(dot)com> wrote:
> In the double write implementation, every checkpoint write is double
> writed,

Unless I'm quite thoroughly confused, which is possible, the double
write will need to happen the first time a buffer is written following
each checkpoint. Which might mean the next checkpoint, but it could
also be sooner if the background writer kicks in, or in the worst case
a buffer has to do its own write.

Furthermore, we can't *actually* write any pages until they are
written *and fsync'd* to the double-write buffer. So the penalty for
the background writer failing to do the right thing is going to go up
enormously. Think about VACUUM or COPY IN, using a ring buffer and
kicking out its own pages. Every time it evicts a page, it is going
to have to doublewrite the buffer, fsync it, and then write it for
real. That is going to make PostgreSQL 6.5 look like a speed demon.
The background writer or checkpointer can conceivably dump a bunch of
pages into the doublewrite area and then fsync the whole thing in
bulk, but a backend that needs to evict a page only wants one page, so
it's pretty much screwed.

--
Robert Haas
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