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. Thanks, -Toshi
