[dm-devel] Snapshot target and DAX-capable devices

[Kani, Toshi]

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