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Re: [HACKERS] WAL logging freezing

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: [HACKERS] WAL logging freezing
Date: 2006-10-30 22:10:08
Message-ID: 20593.1162246208@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> In fact I don't understand what's the point about multiple databases vs.
> a single database.  Surely a checkpoint would flush all buffers in all
> databases, no?

Yeah --- all the ones that are dirty *now*.  Consider the case where you
vacuum DB X, update its datvacuumxid, and don't checkpoint because the
global min didn't advance.  Now you crash, possibly leaving some hint
bits unwritten; but the datvacuumxid change did make it to disk.  After
restart, vacuum DB Y, update its datvacuumxid, and find that the global
min *did* advance.  You checkpoint, and that guarantees that DB Y is
clean for the clog truncation.  But DB X isn't.

The 8.2 changes have created the equivalent risk at the level of each
individual table.  We can't write a vacuumxid change unless we are sure
that the hint-bit changes it promises are actually down to disk.

			regards, tom lane

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