[dm-devel] [PATCH v5 3/3] dm mpath: add CONFIG_DM_MULTIPATH_SG_IO - failover for SG_IO

[Christoph Hellwig]

On Mon, Jun 28, 2021 at 05:15:58PM +0200, mwilck at suse.com wrote:
> The qemu "pr-helper" was specifically invented for it. I
> believe that this is the most important real-world scenario for sending
> SG_IO ioctls to device-mapper devices.

pr-helper obviously does not SG_IO on dm-multipath as that simply does
not work.

More importantly - if you want to use persistent reservations use the
kernel ioctls for that.  These work on SCSI, NVMe and device mapper
without any extra magic.

Failing over SG_IO does not make sense.  It is an interface specically
designed to leave all error handling to the userspace program using it,
and we should not change that for one specific error case.  If you
want the kernel to handle errors for you, use the proper interfaces.
In this case this is the persistent reservation ioctls.  If they miss
some features that qemu needs we should add those.