periodic iowait spike

periodic iowait spike

[Toralf Förster]

The picture in [1], created with isag from the sysstat package, shows a periodic spike in the iowait of my tinderbox server (hardened Gentoo 3.19.6-r1), correlating to write transactions per seconds from a similar graph.

I'm wondering what (kernel, btrfs or ?) is causing this.


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

-- 
Toralf
pgp key: 7B1A 07F4 EC82 0F90 D4C2  8936 872A E508 0076 E94E
--
"; the past is all dirty and cruel in the modern popular imagination, with the exception of the Romans, who are just cruel"
Ian Mortimer, 2008, "The Time Traveller's Guide to Medieval England"

Re: periodic iowait spike

[David Sterba]

On Sun, May 03, 2015 at 12:02:08PM +0200, Toralf Förster wrote:
> The picture in [1], created with isag from the sysstat package, shows
> a periodic spike in the iowait of my tinderbox server (hardened Gentoo
> 3.19.6-r1), correlating to write transactions per seconds from a
> similar graph.
> 
> I'm wondering what (kernel, btrfs or ?) is causing this.

Could be btrfs and the transaction commit, 30 seconds by default. The
plot roughly matches to the ~30 seconds delay, but not everywhere. Raw
numbers would could tell you more. Also, you could try to change the
commit interval (mount option 'commit') and see if the iowait spikes
change as well.

Re: periodic iowait spike

[Toralf Förster]

On 05/06/2015 04:02 PM, David Sterba wrote:
>  Also, you could try to change the
> commit interval (mount option 'commit') and see if the iowait spikes
> change as well.

I changed that option between 10 sec and 299 seconds - same picture.

-- 
Toralf
pgp key: 7B1A 07F4 EC82 0F90 D4C2  8936 872A E508 0076 E94E
--
"; the past is all dirty and cruel in the modern popular imagination, with the exception of the Romans, who are just cruel"
Ian Mortimer, 2008, "The Time Traveller's Guide to Medieval England"

Re: periodic iowait spike

[Toralf Förster]

On 05/03/2015 12:02 PM, Toralf Förster wrote:
> The picture in [1], created with isag from the sysstat package, shows a periodic spike in the iowait of my tinderbox server (hardened Gentoo 3.19.6-r1), correlating to write transactions per seconds from a similar graph.
> 
> I'm wondering what (kernel, btrfs or ?) is causing this.
> 
> 
> [1] http://www.zwiebeltoralf.de/pub/sar_cpu_3rdMay.png
> 
After I deleted from that 3 TB drive (1.2 TB were filled, 1.6 TB were free) about 0.5 TB, the spikes went away.
I'm wondering that such spikes happen although the drive had more than 50 % free space ...


-- 
Toralf
pgp key: 7B1A 07F4 EC82 0F90 D4C2  8936 872A E508 0076 E94E
--
"; the past is all dirty and cruel in the modern popular imagination, with the exception of the Romans, who are just cruel"
Ian Mortimer, 2008, "The Time Traveller's Guide to Medieval England"