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Re: Checkpoints occur too frequently

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: pgsql-bugs(at)postgresql(dot)org, markw(at)osdl(dot)org
Subject: Re: Checkpoints occur too frequently
Date: 2004-12-14 23:31:24
Message-ID: 24551.1103067084@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> DEBUG1 messages showed that there is an apparent limit of 255 xlog files
> per checkpoint -

The volume-based checkpoint trigger code is

                if (IsUnderPostmaster &&
                    (openLogId != RedoRecPtr.xlogid ||
                     openLogSeg >= (RedoRecPtr.xrecoff / XLogSegSize) +
                     (uint32) CheckPointSegments))
                {
#ifdef WAL_DEBUG
                    if (XLOG_DEBUG)
                        elog(LOG, "time for a checkpoint, signaling bgwriter");
#endif
                    RequestCheckpoint(false);
                }

which now that I look at it obviously forces a checkpoint whenever
xlogid (the upper half of XLogRecPtr) changes, ie every 4GB of WAL
output.  I suppose on a high-performance platform it's possible that
one would want checkpoints further apart than that, though the idea
of plowing through multiple gigabytes of WAL in order to recover from
a crash is a bit daunting.

It's not immediately obvious how to recast the comparison without
either creating overflow bugs or depending on 64-bit-int arithmetic
being available.  Thoughts?

			regards, tom lane

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