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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