Re: Auto Vacuum - Mailing list pgsql-hackers

Matthew T. O'Connor looked at this fairly closely leading up to 8.0
feature freeze. There was a long discussion earlier this year with respect
to libpq vs. using backend functions directly to vacuum multiple
databases.

http://archives.postgresql.org/pgsql-hackers/2004-03/msg00931.php

This should be mandatory reading before proceeding. Also, you want to talk
to Matthew and review his patch (submitted early July, I think).

Thanks,

Gavin

On Sun, 28 Nov 2004, Bruce Momjian wrote:

>
> I have added an auto-vacuum TODO item:
>
> * Auto-vacuum
>         o Move into the backend code
>         o Scan the buffer cache to find free space or use background writer
>         o Use free-space map information to guide refilling
>
>
> ---------------------------------------------------------------------------
>
> Russell Smith wrote:
> > Hi All,
> >
> > I am doing serious thinking about the implementation of Auto Vacuum as part of the backend, Not using libpq, but
classinginternal functions directly.
 
> > It appears to me that calling internal functions directly is a better implementation than using the external
libraryto do the job.
 
> >
> > I know I might be stepping on Matthew's toes, but I don't really want to.  I am a complete newbie to the postgresql
code,however I am trying.
 
> > Vacuum appears to be one of the bigger saw points with administrator having to configure it via scheduled tasks.
> >
> > I have outlined things I have thought about below.  I've surely missed a lot, and am open to feedback.  Others may
likethe current tuning used
 
> > by the external autovacuum, however to have stats that are collected at vacuum, and used for later vacuum would
meanwe don't need the
 
> > stats collector running.
> >
> > The major autovacuum issues
> >
> > 1. Transaction Wraparound
> > 2. Vacuum of relations
> > 3. Tracking of when to do vacuums
> > 4. Where to store information needed by auto vacuum
> >
> > I would suggest the first step is to replicate the wraparound and relation vacuuming from the current code first.
ButI could be wrong.
 
> > Currently there are a lot of tuning options in the external autovacuum, and the best way to run vacuum internally
willneed to be thought out.
 
> >
> > 1. Transaction Wraparound
> >
> > It appears from the code, that the best time to do a transaction wraparound db wide vacuum is when the
> > frozenXID has wrapped, TransactionIdPrecedes(myXID, dbform->datfrozenxid).  And is probably the most
> > simple vacuum to implement.
> >
> >
> > 2. Vacuuming of relations
> >
> > Currently, the entire heap must be vacuumed at one time.  I would possible be desireable to have only part of the
relationvacuumed at
 
> > a time.  If you can find out which parts of the relation have the most slack space.  There is a todo item regarding
trackingrecent deletions
 
> > so they can be resused.  Some form of this would be helpful to work out what to vacuum.  Performance issues for
thistype of activity
 
> > may be a concern.  But I have no experience to be able to make comment on them.  So I welcome yours.
> >
> > 3. Tracking of when to vacuum
> >
> > Current autovacuum relies the stats collector to be running.  I would like to only use the stats if they are
available,
> > and have an option to be able to vacuum accurately without having to have stats running.
> >
> > By adding certain histograms, on tuples, filesize and slack space we can guage the time between vacuums
> >
> > number of tuples will show the inserts as opposed to updates.
> > file size will show that the file is growing and by how much between vacuums.
> > slack space will show the delete/updated records.
> >
> > A new guc and relation option would need to be implemented to give a target slack space in a file.
> > this will help to reduce growth in relations if vacuum happens to not run frequently enough.  This information
> > can also inform autovacuum that it should be vacuuming more frequently.  The number would be a percentage,
> > eg 10% of the total file size is allowed to be unused.  Also alter table would allow users to set levels of
slackness
> > for each relation.  If the table gets too much more than the target slack space, a "partial" vacuum full could be
run
> > to reduce the size of the table by moving tuples at the end of the table to slack space near the beginning and
> > shortening the table length.  It would require a full table lock, but you may be able to space it out, to only do a
page
> > at a time.
> >
> > /* target percentage of slack space */
> > vacuum_default_target_slack_space = 10
> >
> > ALTER TABLE SET TARGET SLACK SPACE = 10;
> >
> > 4. Where to store information required by auto vacuum.
> >
> > Auto vacuum needs somewhere to stop the information it needs about current and future vacuuming.  I am unsure of
where
> > to put this.  It appears as if it will have a number of fields.  I feel like pg_class is the best place to put the
information,but
 
> > again I have no idea.
> >
> > That's the best I can do for now.  I can clarify things further if required.
> >
> > Regards
> >
> > Russell.
> >
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>
> --
>   Bruce Momjian                        |  http://candle.pha.pa.us
>   pgman@candle.pha.pa.us               |  (610) 359-1001
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