Merlin Moncure wrote:
>On Mon, Oct 5, 2009 at 5:17 AM, Michal Vitecek <fuf@xxxxxxxx> 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.
Any hints?
Thanks,
--
Michal Vitecek (fuf@xxxxxxxx)
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