https://bugzilla.novell.com/show_bug.cgi?id=816388
https://bugzilla.novell.com/show_bug.cgi?id=816388#c41
--- Comment #41 from Jean Delvare 2013-06-18 09:11:11 UTC ---
I would also add that some power saving settings can be misleading.
For example, on disks which go to sleep very often (ever 8 seconds for
example), the default value of /proc/sys/vm/dirty_expire_centisecs (30 seconds)
makes no sense. When data is being written to the disk, the disk will wake up
every 30 seconds, go to sleep after 8 seconds, wake up again after 22 seconds,
etc. This is counterproductive, as the power savings achieved while sleeping
are lost when waking up / unparking / parking. And performance suffers as well.
While you would think that increasing dirty_expire_centisecs is the standard
method to save more power, here the proper solution would be to lower
dirty_expire_centisecs to, say, 5 seconds. This would guarantee that the disk
only goes to sleep when all data has been written to the disk.
OTOH increasing dirty_expire_centisecs would note help significantly, unless
you make it insanely large (say 30 minutes) but then the risk of data loss
would be increased, and performance might suffer when the writeback finally
triggers.
As a last note, this is unfortunate that the dirty settings are system-wide.
Being able to tune them per disk would be quite useful, methinks.
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