Re: RFC: I/O bandwidth controller

On Fri, 2008-08-08 at 16:20 +0900, Ryo Tsuruta wrote:

quoted

quoted

  - Implement a block layer resource controller. dm-ioband is a working
solution and feature rich but its dependency on the dm infrastructure is
likely to find opposition (the dm layer does not handle barriers
properly and the maximum size of I/O requests can be limited in some
cases). In such a case, we could either try to build a standalone
resource controller based on dm-ioband (which would probably hook into
generic_make_request) or try to come up with something new.

I doubt about the maximum size of I/O requests problem. You can't avoid
this problem as far as you use device mapper modules with such a bad
manner, even if the controller is implemented as a stand-alone controller.
There is no limitation if you only use dm-ioband without any other device
mapper modules.

The following is a part of source code where the limitation comes from.

dm-table.c: dm_set_device_limits()
        /*
         * Check if merge fn is supported.
         * If not we'll force DM to use PAGE_SIZE or
         * smaller I/O, just to be safe.
         */

        if (q->merge_bvec_fn && !ti->type->merge)
                rs->max_sectors =
                        min_not_zero(rs->max_sectors,
                                     (unsigned int) (PAGE_SIZE >> 9));

As far as I can find, In 2.6.27-rc1-mm1, Only some software RAID
drivers and pktcdvd driver define merge_bvec_fn().

Yup, exactly. The implication of this is that we may see a drop in
performance in some RAID configurations.