Re: Spread checkpoint sync

On Thu, Dec 2, 2010 at 2:24 PM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> On Wed, Dec 1, 2010 at 4:25 AM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
>>> I ask because I don't have a mental model of how the pause can help.
>>> Given that this dirty data has been hanging around for many minutes
>>> already, what is a 3 second pause going to heal?
>>>
>>
>> The difference is that once an fsync call is made, dirty data is much more
>> likely to be forced out.  It's the one thing that bypasses all other ways
>> the kernel might try to avoid writing the data
>
> I had always assumed the problem was that other I/O had been done to
> the files in the meantime. I.e. the fsync is not just syncing the
> checkpoint but any other blocks that had been flushed since the
> checkpoint started.

It strikes me that we might start the syncs of the files that the
checkpoint isn't going to dirty further at the start of the
checkpoint, and do the rest at the end.

--
Robert Haas
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