Re: FSM Corruption (was: Could not read block at end of the relation)

On Wed, Mar 13, 2024 at 10:37:21AM +0100, Ronan Dunklau wrote:
> Le mardi 12 mars 2024, 23:08:07 CET Noah Misch a écrit :
> > > I would argue this is fine, as corruptions don't happen often, and FSM is
> > > not an exact thing anyway. If we record a block as full, it will just be
> > > unused until the next vacuum adds it back to the FSM with a correct
> > > value.
> > If this happened only under FSM corruption, it would be fine.  I was
> > thinking about normal extension, with an event sequence like this:
> > 
> > - RelationExtensionLockWaiterCount() returns 10.
> > - Backend B1 extends by 10 pages.
> > - B1 records 9 pages in the FSM.
> > - B2, B3, ... B11 wake up and fetch from the FSM.  In each of those
> > backends, fsm_does_block_exist() returns false, because the cached relation
> > size is stale.  Those pages remain unused until VACUUM re-records them in
> > the FSM.
> > 
> > PostgreSQL added the multiple-block extension logic (commit 719c84c) from
> > hard-won experience bottlenecking there.  If fixing $SUBJECT will lose 1% of
> > that change's benefit, that's probably okay.  If it loses 20%, we should
> > work to do better.  While we could rerun the 2016 benchmark to see how the
> > patch regresses it, there might be a simple fix.  fsm_does_block_exist()
> > could skip the cache when blknumber>=cached, like this:
> > 
> >     return((BlockNumberIsValid(smgr->smgr_cached_nblocks[MAIN_FORKNUM]) 
> &&
> >             blknumber < smgr-
> >smgr_cached_nblocks[MAIN_FORKNUM]) ||
> >            blknumber < RelationGetNumberOfBlocks(rel));
> > 
> > How do you see it?  That still leaves us with an avoidable lseek in B2, B3,
> > ... B11, but that feels tolerable.  I don't know how to do better without
> > switching to the ReadBufferMode approach.

> That makes sense. I'm a bit worried about triggering additional lseeks when we 
> in fact have other free pages in the map, that would pass the test. I'm not 
> sure how relevant it is given the way we search the FSM with fp_next_slot 
> though...

That's a reasonable thing to worry about.  We could do wrong by trying too
hard to use an FSM slot, and we could do wrong by not trying hard enough.

> To address that, I've given a bit of thought about enabling / disabling the 
> auto-repair behaviour with a flag in GetPageWithFreeSpace to distinguish the 
> cases where we know we have a somewhat up-to-date value compared to the case 
> where we don't (as in, for heap, try without repair, then get an uptodate 
> value to try the last block, and if we need to look at the FSM again then ask 
> for it to be repaired) but it brings way too much complexity and would need 
> careful thought for each AM. 
> 
> So please find attached a patch with the change you propose.
> 
> Do you have a link to the benchmark you mention though to evaluate the patch 
> against it ?

Here is one from the thread that created commit 719c84c:
https://www.postgresql.org/message-id/flat/CA%2BTgmob7xED4AhoqLspSOF0wCMYEomgHfuVdzNJnwWVoE_c60g%40mail.gmail.com

There may be other benchmarks earlier in that thread.