On 1/16/21 4:11 PM, Anastasia Lubennikova wrote:
>
> ...
>
> As Pavan correctly figured it out before the problem is that
> RelationGetBufferForTuple() moves to the next page, losing free space in
> the block:
>
> > ... I see that a relcache invalidation arrives
> > after 1st and then after every 32672th block is filled. That clears the
> > rel->rd_smgr field and we lose the information about the saved target
> > block. The code then moves to extend the relation again and thus
> skips the
> > previously less-than-half filled block, losing the free space in that
> block.
>
> The reason of this cache invalidation is vm_extend() call, which happens
> every 32762 blocks.
>
> RelationGetBufferForTuple() tries to use the last page, but for some
> reason this code is under 'use_fsm' check. And COPY FROM doesn't use fsm
> (see TABLE_INSERT_SKIP_FSM).
>
>
> /*
> * If the FSM knows nothing of the rel, try the last page
> before we
> * give up and extend. This avoids one-tuple-per-page syndrome
> during
> * bootstrapping or in a recently-started system.
> */
> if (targetBlock == InvalidBlockNumber)
> {
> BlockNumber nblocks = RelationGetNumberOfBlocks(relation);
> if (nblocks > 0)
> targetBlock = nblocks - 1;
> }
>
>
> I think we can use this code without regard to 'use_fsm'. With this
> change, the number of toast rel pages is correct. The patch is attached.
>
Thanks for the updated patch, this version looks OK to me - I've marked
it as RFC. I'll do a bit more testing, review, and then I'll get it
committed.
regards
--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
