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From: Jon Schewe <jpschewe@mtu.net>
To: Greg Smith <greg@2ndquadrant.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: How filesystems matter with PostgreSQL
Date: Sat, 05 Jun 2010 18:03:59 -0500
Message-ID: <4C0AD7DF.1030705@mtu.net> (raw)
In-Reply-To: <4C0AD547.2040308@2ndquadrant.com>
References: <4C08EEDF.7090403@mtu.net>
	<4C0AD180.90607@2ndquadrant.com>
	<4C0AD280.5020105@mtu.net>
	<4C0AD547.2040308@2ndquadrant.com>



On 06/05/2010 05:52 PM, Greg Smith wrote:
> Jon Schewe wrote:
>>>   If that's the case, what you've measured is which filesystems are
>>> safe because they default to flushing drive cache (the ones that take
>>> around 15 minutes) and which do not (the ones that take >=around 2
>>> hours).  You can't make ext3 flush the cache correctly no matter what
>>> you do with barriers, they just don't work on ext3 the way PostgreSQL
>>> needs them to.
>>>
>>>     
>> So the 15 minute runs are doing it correctly and safely, but the slow
>> ones are doing the wrong thing? That would imply that ext3 is the safe
>> one. But your last statement suggests that ext3 is doing the wrong
>> thing.
>>   
>
> I goofed and reversed the two times when writing that.  As is always
> the case with this sort of thing, the unsafe runs are the fast ones. 
> ext3 does not ever do the right thing no matter how you configure it,
> you have to compensate for its limitations with correct hardware setup
> to make database writes reliable.
>
OK, so if I want the 15 minute speed, I need to give up safety (OK in
this case as this is just research testing), or see if I can tune
postgres better.





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