[lkml]   [2008]   [Jan]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: [patch 6/6] mm: bdi: allow setting a maximum for the bdi dirty limit

    On Thu, 2008-01-31 at 10:46 +0100, Miklos Szeredi wrote:
    > > On Tue, 29 Jan 2008 16:49:06 +0100
    > > Miklos Szeredi <> wrote:
    > >
    > > > Add "max_ratio" to /sys/class/bdi. This indicates the maximum
    > > > percentage of the global dirty threshold allocated to this bdi.
    > >
    > > Maybe I'm having a stupid day, but I don't understand the semantics of this
    > > min and max at all. I've read the code, and I've read the comments (well,
    > > I've hunted for some) and I've read the docs.
    > >
    > > I really don't know how anyone could use this in its current state without
    > > doing a lot of code-reading and complex experimentation. All of which
    > > would be unneeded if this tunable was properly documented.
    > >
    > > So. Please provide adequate documentation for this tunable. I'd suggest
    > > that it be pitched at the level of a reasonably competent system operator.
    > > It should help them understand why the tunable exists, why they might
    > > choose to alter it, and what effects they can expect to see. Hopefully a
    > > reaonably competent kernel developer can then understand it too.
    > OK. I think what's missing from some docs, is a high level
    > description of the per-bdi throttling algorithm, and how it affects
    > writeback. Because with info, I think the min and max ratios are
    > trivially understandable: they just override the result of the
    > algorithm, in case it would mean too high or too low threshold.
    > Peter, could you write something about that?


    How about something like:

    Under normal circumstances each device is given a part of the total
    write-back cache that relates to its current avg writeout speed in
    relation to the other devices.

    min_ratio - allows one to assign a minimum portion of the write-back
    cache to a particular device. This is useful in situations where you
    might want to provide a minimum QoS. (One request for this feature came
    from flash based storage people who wanted to avoid writing out at all
    costs - they of course needed some pdflush hacks as well)

    max_ratio - allows one to assign a maximum portion of the dirty limit to
    a particular device. This is useful in situations where you want to
    avoid one device taking all or most of the write-back cache. Eg. an NFS
    mount that is prone to get stuck, or a FUSE mount which you don't trust
    to play fair.

     \ /
      Last update: 2008-01-31 11:21    [W:0.022 / U:16.224 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site