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Auto Vacuum

From: Russell Smith <mr-russ(at)pws(dot)com(dot)au>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Auto Vacuum
Date: 2004-11-29 02:19:41
Message-ID: 200411291319.41798.mr-russ@pws.com.au (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi All,

I am doing serious thinking about the implementation of Auto Vacuum as part of the backend, Not using libpq, but classing internal functions directly.
It appears to me that calling internal functions directly is a better implementation than using the external library to 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 like the current tuning used
by the external autovacuum, however to have stats that are collected at vacuum, and used for later vacuum would mean we 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. But I could be wrong.
Currently there are a lot of tuning options in the external autovacuum, and the best way to run vacuum internally will need 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 relation vacuumed at
a time.  If you can find out which parts of the relation have the most slack space.  There is a todo item regarding tracking recent deletions
so they can be resused.  Some form of this would be helpful to work out what to vacuum.  Performance issues for this type 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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