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@xxxxxxxx> SUSE Labs, CR -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
