Re: Spin down

[Vincent Pelletier <plr.vincent@gmail.com>]

Le mardi 06 décembre 2011 05:39:23, NeilBrown a écrit :
> Hmm... I cannot reproduce this which makes it harder.

Rebooting on a CONFIG_DYNAMIC_DEBUG-enabled kernel and ssh'ing soon enough, I 
think I found the culprit: noflushd process.
(cc'ing its author)

<noflushd intro>
noflushd is a nice daemon helping suspending disks by tracking disk *read* 
activity. When enough time has passed without reads, it disables automated 
flushing (echo 0 > /proc/sys/vm/dirty_writeback_centisecs) and takes over 
flushing for individual block devices (only flushing spinning devices).
This way, there is no extra delay in writes when disk spins, and only an 
explicit flush (or read) will cause disk to spin up.
</noflushd intro>

I enabled block_trace to see if the problem was still here, and there was no 
activity for 10+ seconds. Then I saw traces for noflushd process, and 
previously-reported writes started ticking every 5s. 5s is the default (at 
least, on my machine) writeback period, so it's what noflushd uses when taking 
over per-device flushing.
I stopped noflushd process, and tada, problem solved.

FWIW, noflushd does individual flushes by opening bock device for write, and 
fsync()'ing it.

Sorry for the noise.

Regards,
-- 
Vincent Pelletier
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