Re: [HACKERS] [PATCH] Vacuum: Update FSM more frequently

From: Claudio Freire <klaussfreire(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, PostgreSQL-Dev <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] [PATCH] Vacuum: Update FSM more frequently
Date: 2018-03-26 17:47:23
Message-ID: CAGTBQpYa7pDsLQO-U=0xGQgJVKku46XOZ74NZdCFeor+mk9+nQ@mail.gmail.com
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On Mon, Mar 26, 2018 at 2:46 PM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
> On Mon, Mar 26, 2018 at 11:26 AM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
>> On Mon, Mar 26, 2018 at 11:19 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
>>>> On Sat, Mar 24, 2018 at 4:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>>> I hadn't paid any attention to this patch previously, so maybe I'm
>>>>> missing something ... but this sure seems like a very bizarre approach
>>>>> to the problem. If the idea is to fix the FSM's upper levels after
>>>>> vacuuming a known sub-range of the table, why would you not proceed
>>>>> by teaching FreeSpaceMapVacuum to recurse only within that sub-range
>>>>> of page numbers? This setup with a threshold seems entirely Rube
>>>>> Goldbergian. It's dependent on a magic threshold number that you can
>>>>> only select by trial and error, and it's inevitably going to spend time
>>>>> updating segments of the FSM tree that have nothing to do with the part
>>>>> that's been vacuumed.
>>>
>>>> Well, the point is to not only update the range we know we've
>>>> vacuumed, but also to finish the updates done by a potential
>>>> previously cancelled autovacuum.
>>>
>>> I think that's not an important consideration, or at least would stop
>>> being one after a patch like this. The reason it's a problem now is
>>> precisely that we don't try to vacuum the FSM till the very end; if
>>> we did FSM cleanup every X pages --- in particular, before not after
>>> the final relation-truncation attempt --- there wouldn't be risk of
>>> skipping so much FSM work that we need to worry about forcing the
>>> issue just in case there'd been a prior cancellation.
>>
>> I'm thinking that in conjunction with the high MWM patch for vacuum,
>> it could still happen that considerable amount of vacuuming is left
>> unexposed upon cancellation.
>>
>> The "bizarre" approach provides some relief.
>>
>> I'll see about implementing the FSM range vacuuming operation for
>> non-initial runs, there seems to be consensus that it's a good idea.
>>
>> But I still think this partial run at the very beginning is useful still.
>
> Attached patches, rebased and modified as discussed:
>
> 1 no longer does tree pruning, it just vacuums a range of the FSM
>
> 2 reintroduces tree pruning for the initial FSM vacuum
>
> 3 and 4 remain as they were, but rebased

Sorry, ignore patch number 3 in the earlier mail, I selected the wrong
patch to attach.

Attached now is the correct number 3

Attachment Content-Type Size
0003-FSM-Fix-relation-extension-FSM-update-v3.patch text/x-patch 2.9 KB

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