On Tue, Oct 6, 2009 at 10:59 AM, Michal Vitecek <fuf@mageo.cz> wrote:
> Merlin Moncure wrote:
>>On Mon, Oct 5, 2009 at 5:17 AM, Michal Vitecek <fuf@mageo.cz> wrote:
>>
>>> Could the problem be the HW RAID card? There's ServerRAID 8k with 256MB
>>> with write-back enabled. Could it be that its internal cache becomes
>>> full and all disk I/O operations are delayed until it writes all
>>> changes to hard drives?
>>
>>that's possible...the red flag is going to be iowait. if your server
>>can't keep up with the sync demands for example, you will eventually
>>outrun the write cache and you can start to see slow queries. With
>>your server though it would take in the hundreds of (write)
>>transactions per second to do that minimum.
>
> The problem is that the server is not loaded in any way. The iowait is
> 0.62%, there's only 72 sectors written/s, but the maximum await that I
> saw was 28ms (!). Any attempts to reduce the time (I/O schedulers,
> disabling bgwriter, increasing number of checkpoints, decreasing shared
> buffers, disabling read cache on the card etc.) didn't help. After some
> 3-5m there occurs a COMMIT which takes 100-10000x longer time than
> usual. Setting fsynch to off Temporarily improved the COMMIT times
> considerably but I fear to have this option off all the time.
>
> Is anybody else using the same RAID card? I suspect the problem lies
> somewhere between the aacraid module and the card. The aacraid module
> ignores setting of the 'cache' parameter to 3 -- this should completely
> disable the SYNCHRONIZE_CACHE command.
I think you're right. One thing you can do is leave fsync on but
disable synchronous_commit. This is compromise between fsync on/off
(data consistent following crash, but you may lose some transactions).
We need to know what iowait is at the precise moment you get the long
commit time. Throw a top, give it short update interval (like .25
seconds), and watch.
merlin
