Re: [PATCH] file-posix: Probe paths and retry SG_IO on potential path errors

[Kevin Wolf <kwolf@redhat.com>]

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Am 13.05.2025 um 15:51 hat Stefan Hajnoczi geschrieben:
> On Tue, May 13, 2025 at 01:37:30PM +0200, Kevin Wolf wrote:
> > When scsi-block is used on a host multipath device, it runs into the
> > problem that the kernel dm-mpath doesn't know anything about SCSI or
> > SG_IO and therefore can't decide if a SG_IO request returned an error
> > and needs to be retried on a different path. Instead of getting working
> > failover, an error is returned to scsi-block and handled according to
> > the configured error policy. Obviously, this is not what users want,
> > they want working failover.
> > 
> > QEMU can parse the SG_IO result and determine whether this could have
> > been a path error, but just retrying the same request could just send it
> > to the same failing path again and result in the same error.
> > 
> > With a kernel that supports the DM_MPATH_PROBE_PATHS ioctl on dm-mpath
> > block devices (queued in the device mapper tree for Linux 6.16), we can
> > tell the kernel to probe all paths and tell us if any usable paths
> > remained. If so, we can now retry the SG_IO ioctl and expect it to be
> > sent to a working path.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  block/file-posix.c | 82 +++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 81 insertions(+), 1 deletion(-)
> 
> Maybe the probability of retry success would be higher with a delay so
> that intermittent issues have time to resolve themselves. Either way,
> the patch looks good.

I don't think adding a delay here would be helpful. The point of
multipath isn't that you wait until a bad path comes back, but that you
just switch to a different path until it is restored.

Ideally, calling DM_MPATH_PROBE_PATHS would just remove all the bad
paths instantaneously and we would either be able to send the request
using one of the remaining good paths, or know that we have to fail. In
practice, the ioctl will probably have to wait for a timeout, so you get
a delay anyway.

What could potentially be useful is a new error policy [rw]error=retry
with a configurable delay. This wouldn't be in file-posix, but in the
devices after file-posix came to the conclusion that there is currently
no usable path. On the other hand, retrying indefinitely is probably not
what you want either, so that could quickly become rather complex.

> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Thanks.

Kevin

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