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From: Alvaro Herrera <alvherre@commandprompt.com>
To: Scott Marlowe <scott.marlowe@gmail.com>
Cc: Matthew Dennis <mdennis@merfer.net>
Cc: david@lang.hm, Matthew Wakeling <matthew@flymine.org>
Cc: PostgreSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: select on 22 GB table causes "An I/O error occured while sending to the backend." exception
Date: Thu, 28 Aug 2008 22:32:18 -0400
Message-ID: <20080829023218.GK8424@alvh.no-ip.org> (raw)
In-Reply-To: <dcc563d10808281901w6d449f52t50e5da7938f1fb25@mail.gmail.com>
References: <alpine.DEB.1.10.0808281411220.4454@aragorn.flymine.org>
	<EADBB336-A14E-492C-A8CA-105C8F77A43A@blighty.com>
	<alpine.DEB.1.10.0808281702450.4454@aragorn.flymine.org>
	<48B6EA7E.2090707@emolecules.com>
	<alpine.DEB.1.10.0808282110140.4454@aragorn.flymine.org>
	<dcc563d10808281542u383808bci4874713a31bd1cc6@mail.gmail.com>
	<alpine.DEB.1.10.0808281602460.2713@asgard.lang.hm>
	<dcc563d10808281811t5c98dcaexd98c5cfe679f428d@mail.gmail.com>
	<e94d85500808281853g22f67274pc9b4c5e3b30bb05a@mail.gmail.com>
	<dcc563d10808281901w6d449f52t50e5da7938f1fb25@mail.gmail.com>

Scott Marlowe escribió:

> scenario 1:  There's a postmaster, it owns all the child processes.
> It gets killed.  The Postmaster gets restarted.  Since there isn't one
> running, it comes up.

Actually there's an additional step required at this point.  There isn't
a postmaster running, but a new one refuses to start, because the shmem
segment is in use.  In order for the second postmaster to start, the
sysadmin must remove the PID file by hand.

> starts new child processes.  Meanwhile, the old child processes that
> don't belong to it are busy writing to the data store.  Instant
> corruption.

In this scenario, it is both a kernel fault and sysadmin stupidity.  The
corruption that ensues is 100% deserved.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support



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