Re: Occasional giant spikes in CPU load

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.

-Dave