Re: xlog.c: WALInsertLock vs. WALWriteLock

Excerpts from Jeff Janes's message of mar oct 26 12:22:38 -0300 2010:

> I don't think that holding WALWriteLock accomplishes much.  It
> prevents part of the buffer from being written out to OS/disk, and
> thus becoming eligible for being overwritten in the buffer, but the
> WALInsertLock prevents it from actually being overwritten.  And what
> if the part of the buffer you want to read was already eligible for
> overwriting but not yet actually overwritten?  WALWriteLock won't
> allow you to safely access it, but WALInsertLock will (assuming you
> have a safe way to identify the record in the first place).  For
> either case, holding it in shared mode would be sufficient.

And horrible for performance, I imagine.  Those locks are highly trafficked.

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Álvaro Herrera <alvherre@commandprompt.com>
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