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    SubjectRe: [PATCH 07/13] writeback: explicit low bound for vm.dirty_ratio
    On Tue, 10 Aug 2010 12:12:06 +0900 (JST)
    KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

    > > Subject: writeback: explicit low bound for vm.dirty_ratio
    > > From: Wu Fengguang <fengguang.wu@intel.com>
    > > Date: Thu Jul 15 10:28:57 CST 2010
    > >
    > > Force a user visible low bound of 5% for the vm.dirty_ratio interface.
    > >
    > > This is an interface change. When doing
    > >
    > > echo N > /proc/sys/vm/dirty_ratio
    > >
    > > where N < 5, the old behavior is pretend to accept the value, while
    > > the new behavior is to reject it explicitly with -EINVAL. This will
    > > possibly break user space if they checks the return value.
    >
    > Umm.. I dislike this change. Is there any good reason to refuse explicit
    > admin's will? Why 1-4% is so bad? Internal clipping can be changed later
    > but explicit error behavior is hard to change later.

    As a data-point, I had a situation a while back where I needed a value below
    1 to get desired behaviour. The system had lots of RAM and fairly slow
    write-back (over NFS) so a 'sync' could take minutes.

    So I would much prefer allowing not only 1-4, but also fraction values!!!

    I can see no justification at all for setting a lower bound of 5. Even zero
    can be useful - for testing purposes mostly.

    NeilBrown

    > personally I prefer to
    > - accept all value, or
    > - clipping value in dirty_ratio_handler
    >
    > Both don't have explicit ABI change.
    >
    > Thanks.
    >
    >
    >
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