| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
| Cc: | Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Martijn van Oosterhout <kleptog(at)svana(dot)org>, Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>, 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 17:37:26 |
| Message-ID: | 21582.1227029846@sss.pgh.pa.us |
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Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> Alvaro Herrera wrote:
>> Right now we flip hint bits without taking any kind
>> of lock on the page.
> That's not quite true. You need to hold a shared lock on heap page to
> examine the visibility of a tuple, and that's when the hint bits are
> set. So we always hold at least a shared lock on the page while hint
> bits are set.
Right, but we couldn't let hint-bit-setters update the page LSN with
only shared lock. Too much chance of ending up with a scrambled LSN
value.
Could we arrange for the actual LSN-updating to be done while still
holding WALInsertLock? Then we'd be depending on that lock, not the
page-level locks, to serialize. It's not great to be pushing more work
inside that global lock, but it's not very much more work ...
regards, tom lane
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