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Re: select on 22 GB table causes "An I/O error occured while sending to the backend." exception

From: Craig James <craig_james(at)emolecules(dot)com>
To: PostgreSQL Performance <pgsql-performance(at)postgresql(dot)org>
Cc: Matthew Wakeling <matthew(at)flymine(dot)org>
Subject: Re: select on 22 GB table causes "An I/O error occured while sending to the backend." exception
Date: 2008-08-28 18:12:14
Message-ID: 48B6EA7E.2090707@emolecules.com (view raw or flat)
Thread:
Lists: pgsql-performance
Matthew Wakeling wrote:
> On Thu, 28 Aug 2008, Steve Atkins wrote:
>>> Probably the best solution is to just tell the kernel somehow to 
>>> never kill the postmaster.
>>
>> Or configure adequate swap space?
> 
> Oh yes, that's very important. However, that gives the machine the 
> opportunity to thrash.

No, that's where the whole argument for allowing overcommitted memory falls flat.

The entire argument for allowing overcommitted memory hinges on the fact that processes *won't use the memory*.  If they use it, then overcommitting causes problems everywhere, such as a Postmaster getting arbitrarily killed.

If a process *doesn't* use the memory, then there's no problem with thrashing, right?

So it never makes sense to enable overcommitted memory when Postgres, or any server, is running.

Allocating a big, fat terabyte swap disk is ALWAYS better than allowing overcommitted memory.  If your usage is such that overcommitted memory would never be used, then the swap disk will never be used either.  If your processes do use the memory, then your performance goes into the toilet, and you know it's time to buy more memory or a second server, but in the mean time your server processes at least keep running while you kill the rogue processes.

Craig

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Subject: Re: select on 22 GB table causes "An I/O error occured while sending to the backend." exception
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