Re: Snapshot target and DAX-capable devices

[Jan Kara <jack@suse.cz>]

On Mon 27-08-18 16:43:28, Kani, Toshi wrote:
> On Mon, 2018-08-27 at 18:07 +0200, Jan Kara wrote:
> > Hi,
> > 
> > I've been analyzing why fstest generic/081 fails when the backing device is
> > capable of DAX. The problem boils down to the failure of:
> > 
> > lvm vgcreate -f vg0 /dev/pmem0
> > lvm lvcreate -L 128M -n lv0 vg0
> > lvm lvcreate -s -L 4M -n snap0 vg0/lv0
> > 
> > The last command fails like:
> > 
> >   device-mapper: reload ioctl on (253:0) failed: Invalid argument
> >   Failed to lock logical volume vg0/lv0.
> >   Aborting. Manual intervention required.
> > 
> > And the core of the problem is that volume vg0/lv0 is originally of
> > DM_TYPE_DAX_BIO_BASED type but when the snapshot gets created, we try to
> > switch it to DM_TYPE_BIO_BASED because now the device stops supporting DAX.
> > The problem seems to be introduced by Ross' commit dbc626597 "dm: prevent
> > DAX mounts if not supported".
> > 
> > The question is whether / how this should be fixed. The current inability
> > to create snapshots of DAX-capable devices looks weird and the cryptic
> > failure makes it even worse (it took me quite a while to understand what is
> > failing and why). OTOH I see the rationale behind Ross' change as well.
> 
> Here are the dm-snap changes that went along with the original DAX
> support.
> 
> commit b5ab4a9ba55
> commit f6e629bd237
> 
> Basically, snapshots can be added/removed to DAX-capable devices, but
> snapshots need to be mounted without dax option.

Yes, and after these two commits things were working. But then commit
dbc626597 broke things again so currently snapshotting DAX-capable devices
does not work. Just try with 4.18...

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR