Re: Occasional giant spikes in CPU load

On Wed, Apr 7, 2010 at 10:50 PM, David Rees <drees76(at)gmail(dot)com> wrote:
> On Wed, Apr 7, 2010 at 7:06 PM, Craig James <craig_james(at)emolecules(dot)com> wrote:
>> On 4/7/10 5:47 PM, Robert Haas wrote:
>>> On Wed, Apr 7, 2010 at 6:56 PM, David Rees<drees76(at)gmail(dot)com>  wrote:
>>>>> synchronous_commit = off
>>>>
>>>> You are playing with fire here.  You should never turn this off unless
>>>> you do not care if your data becomes irrecoverably corrupted.
>>>
>>> That is not correct.  Turning off synchronous_commit is sensible if
>>> you don't mind losing the last few transactions on a crash.  What will
>>> corrupt your database is if you turn off fsync.
>
> Whoops, you're right.
>
>> A bit off the original topic, but ...
>>
>> I set it this way because I was advised that with a battery-backed RAID
>> controller, this was a safe setting.  Is that not the case?
>
> Robert has it right - with synchronous_commit off, your database will
> always be consistent, but you may lose transactions in the event of a
> crash.
>
> Doesn't matter if you have a BBU or not - all the BBU does is give the
> controller the ability to acknowledge a write without the data
> actually having been written to disk.
>
> According to the documentation, with synchronous_commit off and a
> default wal_writer_delay of 200ms, it's possible to lose up to a
> maximum of 600ms of data you thought were written to disk.

So, IOW, if you're running a social networking web site and your
database is full of status updates sent by teenagers to other
teenagers, you might judge that turning off synchronous_commit is a
reasonable thing to do, if you need the performance. If you're
running a bank and your database is full of information on wire
transfers sent and received, not so much.

...Robert