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    SubjectRe: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio
    On Fri, 20 Aug 2010 15:50:54 +1000
    Con Kolivas <kernel@kolivas.org> wrote:

    > On Fri, 20 Aug 2010 02:13:25 pm KOSAKI Motohiro wrote:
    > > > The dirty_ratio was silently limited to >= 5%. This is not a user
    > > > expected behavior. Let's rip it.
    > > >
    > > > It's not likely the user space will depend on the old behavior.
    > > > So the risk of breaking user space is very low.
    > > >
    > > > CC: Jan Kara <jack@suse.cz>
    > > > CC: Neil Brown <neilb@suse.de>
    > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    > >
    > > Thank you.
    > > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    >
    > I have tried to do this in the past, and setting this value to 0 on some
    > machines caused the machine to come to a complete standstill with small
    > writes to disk. It seemed there was some kind of "minimum" amount of data
    > required by the VM before anything would make it to the disk and I never
    > quite found out where that blockade occurred. This was some time ago (3 years
    > ago) so I'm not sure if the problem has since been fixed in the VM since
    > then. I suggest you do some testing with this value set to zero before
    > approving this change.
    >

    If it is appropriate to have a lower limit, that should be imposed where
    the sysctl is defined in kernel/sysctl.c, not imposed after the fact where
    the value is used.

    As we now have dirty_bytes which over-rides dirty_ratio, there is little
    cost in having a lower_limit for dirty_ratio - it could even stay at 5% -
    but it really shouldn't be silent. Writing a number below the limit to the
    sysctl file should fail.

    NeilBrown


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