Re: Massive I/O spikes during checkpoint

On 7/9/2012 11:14 PM, Maxim Boguk wrote:
>
>
> On Tue, Jul 10, 2012 at 4:03 PM, David Kerr <dmk(at)mr-paradox(dot)net
> <mailto:dmk(at)mr-paradox(dot)net>> wrote:
>
>
> On Jul 9, 2012, at 10:51 PM, Maxim Boguk wrote:
>
>>
>>
>> But what appears to be happening is that all of the data is
>> being written out at the end of the checkpoint.
>>
>> This happens at every checkpoint while the system is under load.
>>
>> I get the feeling that this isn't the correct behavior and
>> i've done something wrong.
>>
>>
>>
>> It's not an actual checkpoints.
>> It's is a fsync after checkpoint which create write spikes hurting
>> server.
>> You should set sysctl vm.dirty_background_bytes and vm.dirty_bytes
>> to reasonable low values
>
> So use bla_bytes instead of bla_ratio?
>
>
> Yes because on 256GB server
> echo 10 > /proc/sys/vm/dirty_ratio
> is equivalent to 26Gb dirty_bytes
>
> and
> echo 5 >/proc/sys/vm/dirty_background_ratio
> is equivalent to 13Gb dirty_background_bytes
>
> It is really huge values.
<sigh> yeah, I never bothered to think that through.

> So kernel doesn't start write any pages out in background before it has
> at least 13Gb dirty pages in kernel memory.
> And at end of the checkpoint kernel trying flush all dirty pages to disk.
>
> Even echo 1 >/proc/sys/vm/dirty_background_ratio is too high value for
> contemporary server.
> That is why *_bytes controls added to kernel.

Awesome, Thanks.