On Mon, Jun 18 2012 at 9:57pm -0400,
Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Mon, Jun 18, 2012 at 11:33:50PM +0200, Spelic wrote:
> > Please note that since I am above MD raid5 (I believe this is the
> > reason), the passdown of discards does not work, as my dmesg says:
> > [160508.497879] device-mapper: thin: Discard unsupported by data
> > device (dm-1): Disabling discard passdown.
> > but AFAIU, unless there is a thinp bug, this should not affect the
> > unmapping of thin blocks by fstrimming xfs... and in fact ext4 is
> > able to do that.
> Does ext4 report that same error?
That message says the underlying device doesn't support discards
(because it is an MD device). But the thinp device still has discards
enabled -- it just won't pass the discards down to the underlying data
So yes, it'll happen with ext4 -- it is generated when the thin-pool
device is loaded (which happens independent of the filesystem that is
The discards still inform the thin-pool that the corresponding extents
are no longer allocated.