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Re: Performance problem with mysql on a 3ware 1+0 raid array

Neil, Thanks for the info but I am kinda confused as to why the sudden
writeout occurs at 4 minutes if the default ext3 settings is to flush
the journal every 5 seconds. Do you know what the correlation between
these times would be. 

I have seen previous posts from you in which you described your
fileservers also having write stalls at around 5 minute intervals. Did
you figure out why things would go bad at 5 minutes interval. There
seems to be no default writeout every 5 minutes

Regards, Yusuf

> > Hi all,
> > 
> > We are observing a consistent interval of about 4 minutes at which there 
> > are large sustained writes to disk that causes mysqld to block and not 
> > respond for the entire period.
> > 
> > We are using data=journal with a 128M journal and the filesystem is 
> > 150GB in size.
> Yep.  This is due to some lazy code in ext3.
> If the journal fills up, then it flushes the whole journal before
> continuing.
> There are three ways to get around this problem.
>  1/ fix the code :-)
>  2/ use a bigger journal.  There is a drawback with bigger journals
>    though as replay after a crash will take longer.
>  3/ Push the bdflush parameters down so that data in the journal will
>     be flushed out more often and the journal will not get full.
>    Something like:
>         echo 40 0 0 0 60 300 60 0 0 > /proc/sys/vm/bdflush
>     The 5th and 6th numbers are significant.
>     The defaults are 500 and 3000 (hundreths of a second) so
>     every 5 seconds it flushs data older than 30 seconds.  If your
>     journal cannot hold 30 seconds of data, this is a problem.
>     So drop the 3000 to maybe 300 or 500 (3 or 5 seconds) and then
>     drop the 500 to a reasonable fraction of that (50  or so (half a
>     second)). 
> 2 and 3 are complimentary.  The bigger the journal, the larger the
> age of flushed buffers can be.

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