From: Shane Ambler <pgsql@Sheeky.Biz>
To: Ivan Voras <ivoras@geri.cc.fer.hr>
Cc: pgsql-performance@postgresql.org
Subject: Re: How to allocate 8 disks
Date: Wed, 05 Mar 2008 01:34:32 +1030
Message-ID: <47CD6500.9030802@Sheeky.Biz> (raw)
In-Reply-To: <fqj920$f03$1@ger.gmane.org>
References: <1204285894.11773.33.camel@franck-gusty>
<dcc563d10802290613v482f52dr43ea95b52886de31@mail.gmail.com>
<1204296705.11773.51.camel@franck-gusty>
<Pine.GSO.4.64.0802291019570.28452@westnet.com>
<47C88826.4040209@paradise.net.nz>
<47C99B3E.4010606@emolecules.com>
<20080301130347.0832b323@commandprompt.com>
<fqj920$f03$1@ger.gmane.org>
Ivan Voras wrote:
> Joshua D. Drake wrote:
>
>> This scares me... You lose WAL you are a goner. Combine your OS and
>> WAL into a RAID 1.
>
> Can someone elaborate on this? From the WAL concept and documentation at
> http://www.postgresql.org/docs/8.3/interactive/wal-intro.html I'd say
> the only data that should be lost are the transactions currently in the
> log but not yet transferred to permanent storage (database files proper).
>
The log records what changes are made to your data files before the data
files are changed. (and gets flushed to disk before the data files are
changed)
In the event of power loss right in the middle of the data files being
updated for a transaction, when power is restored, how do we know what
changes were made to which data files and which changes are incomplete?
Without the log files there is no way to be sure your data files are not
full of "half done transactions"
Chances are that 90% of the time everything is fine but without the log
files how do you check that your data files are as they should be.
(or do you expect to restore from backup after any power outs?)
Keeping them on a raid 1 gives you a level of redundancy to get you past
hardware failures that happen at the wrong time. (as they all do)
--
Shane Ambler
pgSQL (at) Sheeky (dot) Biz
Get Sheeky @ http://Sheeky.Biz
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To: pgsql-performance@postgresql.org
Cc: pgsql@Sheeky.Biz, ivoras@geri.cc.fer.hr
Subject: Re: How to allocate 8 disks
In-Reply-To: <47CD6500.9030802@Sheeky.Biz>
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