Re: [HACKERS] Proposal : For Auto-Prewarm.

On Sun, Feb 26, 2017 at 10:16 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Feb 22, 2017 at 11:49 PM, Mithun Cy <mithun.cy@enterprisedb.com> wrote:
>> Hi all thanks,
>> I have tried to fix all of the comments given above with some more
>> code cleanups.
>
> While reading this patch tonight, I realized a serious problem with
> the entire approach, which is that this patch is supposing that we can
> read relation blocks for every database from a single worker that's
> not connected to any database.  I realize that I suggested that
> approach, but now I think it's broken, because the patch isn't taking
> any locks on the relations whose pages it is reading, and that is
> definitely going to break things.  While autoprewarm is busy sucking
> blocks into the shared buffer cache, somebody could be, for example,
> dropping one of those relations.  DropRelFileNodesAllBuffers and
> friends expect that nobody is going to be concurrently reading blocks
> back into the buffer cache because they hold AccessExclusiveLock, and
> they assume that anybody else who is touching it will hold at least
> AccessShareLock.  But this violates that assumption, and probably some
> others.
>
> This is not easy to fix.  The lock has to be taken based on the
> relation OID, not the relfilenode, but we don't have the relation OID
> in the dump file, and recording it there won't help, because the
> relfilenode can change under us if the relation is rewritten with
> CLUSTER or VACUUM FULL or relevant forms of ALTER TABLE.  I don't see
> a solution other than launching a separate worker for each database,
> which seems like it could be extremely expensive if there are many
> databases.  Also, I am pretty sure it's no good to take locks before
> recovery reaches a consistent state.

So we should move this loading of blocks once the recovery reaches a
consistent state so that we can connect to a database.  To allow
worker, to take a lock, we need to dump relation oid as well.  Is that
what you are envisioning to fix this problem?

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com