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.