[Devel] Re: RFC: I/O bandwidth controller

[Ryo Tsuruta]

Hi,

> >   - 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().

Thanks,
Ryo Tsuruta
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