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Re: [COMMITTERS] pgsql: Create a GUC parametertemp_tablespacesthat allows selection of

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: [COMMITTERS] pgsql: Create a GUC parametertemp_tablespacesthat allows selection of
Date: 2007-06-04 19:34:14
Message-ID: 23390.1180985654@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers
"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> On Mon, 2007-06-04 at 14:41 -0400, Tom Lane wrote:
>> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
>>> The original ideal implementation was to use round-robin/cyclic
>>> selection, which allows much better usage in the above case.
>> 
>> Really?  What if multiple backends are all hitting the same tablespaces
>> in the same order?  A random selection seems much less likely to risk
>> having any self-synchronizing behavior.

> I'd like a single backend to never reuse a temp tablespace that is
> actively being used so that large queries won't randomly conflict with
> themselves. That's pretty certain to draw complaints, IMHO.

> We can do this two ways
> - cycle thru temp tablespaces, as originally suggested (not by me...)
> - pick a random tablespace **other than ones already in active use**

Idea 2 fails as soon as you have more temp files than tablespaces, and
also requires tracking which tablespaces are currently in use, a bit of
complexity we do not have in there.

Perhaps a reasonable compromise could work like this: at the first point
in a transaction where a temp file is created, choose a random list
element, and thereafter advance cyclically for the duration of that
transaction.  This ensures within-transaction spread-out while still
having some randomness across backends.

The reason I'm thinking per-transaction is that we could tie this to
setting up a cached list of tablespace OIDs, which would avoid the
overhead of repeat parsing and tablespace validity checking.  We had
rejected using a long-lived cache because of the problem of tablespaces
getting dropped, but I think one that lasts only across a transaction
would be OK.

And the reason I'm thinking a cache is important is that if you really
want to get any win from this idea, you need to spread the temp files
across tablespaces *per file*, which is not the way it works now.
As committed, the code selects one temp tablespace per sort or hashjoin.
The submitted patch already did it that way for sorts, and I forced the
same for hashjoins, because I wanted to be sure to minimize the number
of executions of aforesaid parsing/checking.  So really that patch is
entirely wrong, and selection of the tablespace for a temp file needs
to be pushed much further down.

Assuming, that is, that you think this point is important enough to
drive the whole design; which I find rather questionable in view of the
fact that the submitted patch contained no mention whatever of any such
consideration.  Or is this just another way in which its documentation
was not up to snuff?

			regards, tom lane

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