Re: Issues with two-server Synch Rep

<div dir="ltr"><br /><div class="gmail_quote">On Mon, Oct 11, 2010 at 7:16 PM, Jeff Davis <span dir="ltr"><<a
href="mailto:pgsql@j-davis.com">pgsql@j-davis.com</a>></span>wrote:<br /><blockquote class="gmail_quote"
style="margin:0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div class="im">On Mon,
2010-10-11at 16:07 -0400, Robert Haas wrote:<br /> > >> It would be far better if we could decouple master
cleanupfrom<br /> > >> standby cleanup, so that only the machine that actually has the old<br /> > >>
querygets bloated.  However, no one seems excited about writing that<br /> > >> code.<br /> > ><br />
>> That doesn't seem just a matter of code, it seems like a major design<br /> > > conflict.<br /> ><br
/>> Yes.  I had the idea of trying to fix this by allowing the standby to<br /> > retain old versions of entire
pagesthat got cleaned up on the master,<br /> > until the transactions that might want to read the old pages were<br
/>> gone.  But that may be prohibitively difficult, not sure.<br /><br /></div>I think you'd end up having a notion
ofa snapshot of block information<br /> (like a FS with snapshots) inside of postgres.<br /><br /> Sounds like a lot of
complexityto me, and the only benefit I see is<br /> moving bloat from the primary to the standby. Granted, that would
be<br/> nice, but I would expect some costs aside from just the complexity.<br /></blockquote></div><br clear="all" />I
hadthis idea when HS was being initially implemented, and I saw two problems with it.<br /><br />.) Identifying block
versionsin buffers. Our XID based MVCC like scheme would work, but we already have a block header which we would want
totrim.<br /><br />.) Effectively every block cleanup will have to create a copy of the block because more often than
notthere's at least one transaction in the system, and when it asks, we need to hand it the old version.<br /><br />
Regards,<br/> -- <br />gurjeet.singh<br />@ EnterpriseDB - The Enterprise Postgres Company<br /><a
href="http://www.EnterpriseDB.com">http://www.EnterpriseDB.com</a><br/><br />singh.gurjeet@{ gmail | yahoo }.com<br
/>Twitter/Skype:singh_gurjeet<br /><br />Mail sent from my BlackLaptop device<br /></div>