[linux-lvm] move dm-integrity metadata to another PV

[David Teigland]

On Sun, Apr 04, 2021 at 08:18:56PM +0200, Sebastian Bachmann wrote:
> Hello,
> 
> I played around with the new dm-integrity integration in lvm. Unfortunately, it
> is very slow, as the checksums has to be written and read on - which is the
> price to pay obviously.
> 
> I thought that it might be a good idea to move the metadata to a fast disk,
> i.e., a SSD, however that is not possible and I get an error message on pvmove:
> 
> # pvmove -n r10_int_rimage_0_imeta /dev/sdd /dev/sda2
>   Unable to pvmove device used for raid with integrity.
> 
> I could not find a reason why this should not be done in theory, thus I guess
> that this is simply not supported by LVM right now?
> Or is there another reason why you should keep the metadata always on the same
> device?

The original implementation allowed a specific device, e.g. an ssd, to
hold all the integrity metadata.  Integrity metadata for all raid images
lived on one device, so there was some doubt that anyone would want to use
it, and the option was dropped.  That option could also be used with
linear+integrity.  Without raid, corrupt data found by integrity couldn't
be recovered, so linear+integrity was also disabled.  Given those
limitations, I'm curious how useful you'd find a dedicated integrity
metadata device, and/or using integrity with a linear LV?

Dave