From: | Aidan Van Dyk <aidan(at)highrise(dot)ca> |
---|---|
To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | Martijn van Oosterhout <kleptog(at)svana(dot)org>, Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gregory Stark <stark(at)enterprisedb(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Block-level CRC checks |
Date: | 2008-11-18 17:34:06 |
Message-ID: | 20081118173406.GE31053@yugib.highrise.ca |
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Lists: | pgsql-hackers |
* Alvaro Herrera <alvherre(at)commandprompt(dot)com> [081118 12:25]:
> I don't think it's a matter of hoy many writes or how much IO. The
> question is locks. Right now we flip hint bits without taking any kind
> of lock on the page. If we're going to WAL-log each hint bit change,
> then we will need to lock the page to update the LSN. This will make
> changing a hint bit a very expensive operation, and maybe a possible
> cause for deadlocks.
Ya, that's obviously the worst option.
> What my patch did was log hint bits in bulk. The problem of that
> approach was precisely that it was not locking the logged page enough
> (locking before setting the "this page needs hint bits logged" bit). Of
> course, the trivial solution is just to lock the page before flipping
> hint bits, but I don't know (and I doubt) whether it would really work
> at all.
But why can't you wal-log the hint bits from the "buffered" page. then your
consitent. At least as consistent as the original write was.
So you're CRC ends up being:
Buffer the page
Calculate CRC on the buffered page
WAL (in bulk) the hint bits (and maybe CRC?)
write buffered page
--
aidan van dyk create like a god,
aidan(at)highrise(dot)ca command like a king,
http://www.highrise.ca/ work like a slave.
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