Re: Replication Syatem

On Tue, 29 Apr 2008, Gauri Kanekar wrote:

> We do vacuum full, as vacuum verbose analyse dont regain space for us.

Ah, now we're getting to the root of your problem here.  You expect that 
VACUUM should reclaim space.

Whenever you UPDATE a row, it writes a new one out, then switches to use 
that version.  This leaves behind the original.  Those now unused rows are

what VACUUM gathers, but it doesn't give that space back to the operating 
system.

The model here assumes that you'll need that space again for the next time

you UPDATE or INSERT a row.  So instead VACUUM just keeps those available 
for database reuse rather than returning it to the operating system.

Now, if you don't VACUUM frequently enough, this model breaks down, and 
the table can get bigger with space that may never get reused.  The idea 
is that you should be VACUUMing up now unneeded rows at about the same 
rate they're being re-used.  When you don't keep up, the database can 
expand in space that you don't get back again.  The right answer to this 
problem is not to use VACUUM FULL; it's to use regular VACUUM more often.

--
* Greg Smith gsmith@[EMAIL PROTECTED]
 http://www.gregsmith.com
Baltimore, MD

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