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[solved] periodic iowait spike at my tinderbox
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toralf
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Joined: 01 Feb 2004
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Location: Hamburg

PostPosted: Sun May 03, 2015 9:07 am    Post subject: [solved] periodic iowait spike at my tinderbox Reply with quote

The picture in [1], created with isag from the sysstat package, shows a periodic spike in the iowait of my tinderbox, correlating to write transactions per seconds from a similar graph.
/me wonders what's (kernel, btrfs) causing this.



[1] http://www.zwiebeltoralf.de/pub/sar_cpu_3rdMay.png


Last edited by toralf on Wed May 13, 2015 6:46 pm; edited 2 times in total
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NeddySeagoon
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PostPosted: Sun May 03, 2015 9:25 am    Post subject: Reply with quote

toralf,

iowait is a busy wait. While the CPU is busy waiting, its not available for other things.
A few years ago AMD64 had a kernel bug related to iowait, which when you hit made it all very slow as iowaits took far too long.

The correlation you see is expected. I can't say if the iowaits are excessive though.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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