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Re: VACUUMs and WAL

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Hannu Krosing <hannu(at)2ndQuadrant(dot)com>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: VACUUMs and WAL
Date: 2008-10-28 12:28:01
Message-ID: 49070551.1090903@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hannu Krosing wrote:
> On Tue, 2008-10-28 at 10:10 +0000, Simon Riggs wrote:
>> On Tue, 2008-10-28 at 11:45 +0200, Hannu Krosing wrote:
>>> On Tue, 2008-10-28 at 08:49 +0000, Simon Riggs wrote:
>>>> Looking at a VACUUM's WAL records makes me think twice about the way we
>>>> issue a VACUUM.
>>>>
>>>> 1. First we scan the heap, issuing a HEAP2 clean record for every block
>>>> that needs cleaning.
>>> IIRC the first heap pass just collects info and does nothing else. 
>>> Is this just an empty/do-nothing WAL record ?
>> 8.3 changed that; it used to work that way. I guess I never looked at
>> the amount of WAL being generated.
> 
> I can't see how it is safe to do anything more than just lookups on
> first pass. 

What's done in the first pass is the same HOT pruning that is done 
opportunistically on other page accesses as well. IIRC it's required for 
correctness, though I can't remember what exactly the issue was.

I don't think the extra WAL volume is a problem; VACUUM doesn't generate 
much WAL, anyway. As for the extra data page writes it causes; yeah, 
that might cause some I/O that could be avoided, but remember that the 
first pass often dirties buffers anyway to set hint bits.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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