On 12 December 2014 at 21:40, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Dec 12, 2014 at 1:51 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> What I don't understand is why we aren't working on double buffering,
>> since that cost would be paid in a background process and would be
>> evenly spread out across a checkpoint. Plus we'd be able to remove
>> FPWs altogether, which is like 100% compression.
>
> The previous patch to implement that - by somebody at vmware - was an
> epic fail. I'm not opposed to seeing somebody try again, but it's a
> tricky problem. When the double buffer fills up, then you've got to
> finish flushing the pages whose images are stored in the buffer to
> disk before you can overwrite it, which acts like a kind of
> mini-checkpoint. That problem might be solvable, but let's use this
> thread to discuss this patch, not some other patch that someone might
> have chosen to write but didn't.
No, I think its relevant.
WAL compression looks to me like a short term tweak, not the end game.
On that basis, we should go for simple and effective, user-settable
compression of FPWs and not spend too much Valuable Committer Time on
it.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services