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    SubjectRe: [dm-devel] [PATCH] dm: Make MIN_IOS, et al, tunable via sysctl.
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    On Tue, 2013-08-20 at 17:22 -0400, Mikulas Patocka wrote:
    > On Fri, 16 Aug 2013, Frank Mayhar wrote:
    > > The device mapper and some of its modules allocate memory pools at
    > > various points when setting up a device. In some cases, these pools are
    > > fairly large, for example the multipath module allocates a 256-entry
    > > pool and the dm itself allocates three of that size. In a
    > > memory-constrained environment where we're creating a lot of these
    > > devices, the memory use can quickly become significant. Unfortunately,
    > > there's currently no way to change the size of the pools other than by
    > > changing a constant and rebuilding the kernel.
    > I think it would be better to set the values to some low value (1 should
    > be enough, there is 16 in some places that is already low enough). There
    > is no need to make it user-configurable, because the user will see no
    > additional benefit from bigger mempools.
    >
    > Maybe multipath is the exception - but other dm targets don't really need
    > big mempools so there is no need to make the size configurable.

    The point is not to make the mempools bigger, the point is to be able to
    make them _smaller_ for memory-constrained environments. In some cases,
    even 16 can be too big, especially when creating a large number of
    devices.

    In any event, it seems unfortunate to me that these values are
    hard-coded. One shouldn't have to rebuild the kernel to change them,
    IMHO.
    --
    Frank Mayhar
    310-460-4042



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