Message-ID: <4C0AE6AC.9080501@mtu.net>
Date: Sat, 05 Jun 2010 19:07:08 -0500
From: Jon Schewe <jpschewe@mtu.net>
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To: Scott Marlowe <scott.marlowe@gmail.com>
CC: Greg Smith <greg@2ndquadrant.com>, pgsql-performance@postgresql.org
Subject: Re: How filesystems matter with PostgreSQL
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	<AANLkTinRedcd5JaZRNphubdrVPlz7NXBkxgjeg4GLV0J@mail.gmail.com>
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On 06/05/2010 07:02 PM, Scott Marlowe wrote:
> On Sat, Jun 5, 2010 at 5:58 PM, Jon Schewe <jpschewe@mtu.net> wrote:
>   
>> On 06/05/2010 06:54 PM, Scott Marlowe wrote:
>>     
>>> On Sat, Jun 5, 2010 at 5:03 PM, Jon Schewe <jpschewe@mtu.net> wrote:
>>>
>>>       
>>>> 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.
>>>>
>>>>         
>>> Or use a trustworthy hardware caching battery backed RAID controller,
>>> either in RAID mode or JBOD mode.
>>>
>>>       
>> Right, because the real danger here is if the power goes out you can end
>> up with a scrambled database, correct?
>>     
> Correct.  Assuming you can get power applied again before the battery
> in the RAID controller dies, it will then flush out its cache and your
> data will still be coherent.
>   
Or if you really don't care if your database is scrambled after a power
outage you can go without the battery backed RAID controller.