Re: Heap truncation without AccessExclusiveLock (9.4)

On 2013-05-17 10:45:26 +0300, Heikki Linnakangas wrote:
> On 16.05.2013 04:15, Andres Freund wrote:
> >On 2013-05-15 18:35:35 +0300, Heikki Linnakangas wrote:
> >>Truncating a heap at the end of vacuum, to release unused space back to
> >>the OS, currently requires taking an AccessExclusiveLock. Although it's only
> >>held for a short duration, it can be enough to cause a hiccup in query
> >>processing while it's held. Also, if there is a continuous stream of queries
> >>on the table, autovacuum never succeeds in acquiring the lock, and thus the
> >>table never gets truncated.
> >>
> >>I'd like to eliminate the need for AccessExclusiveLock while truncating.
> >
> >Couldn't we "just" take the extension lock and then walk backwards from
> >the rechecked end of relation ConditionalLockBufferForCleanup() the
> >buffers?
> >For every such locked page we check whether its still empty. If we find
> >a page that we couldn't lock, isn't empty or we already locked a
> >sufficient number of pages we truncate.
>
> You need an AccessExclusiveLock on the relation to make sure that after you
> have checked that pages 10-15 are empty, and truncated them away, a backend
> doesn't come along a few seconds later and try to read page 10 again. There
> might be an old sequential scan in progress, for example, that thinks that
> the pages are still there.

But that seems easily enough handled: We know the current page in its
scan cannot be removed since its pinned. So make
heapgettup()/heapgetpage() pass something like RBM_IFEXISTS to
ReadBuffer and if the read fails recheck the length of the relation
before throwing an error.

There isn't much besides seqscans that can have that behaviour afaics:
- (bitmap)indexscans et al. won't point to completely empty pages
- there cannot be a concurrent vacuum since we have the appropriate
locks
- if a trigger or something else has a tid referencing a page there need
to be unremovable tuples on it.

The only thing that I immediately see are tidscans which should be
handleable in a similar manner to seqscans.

Sure, there are some callsites that need to be adapted but it still
seems noticeably easier than what you proposed upthread.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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