On 7/10/25 20:10, Benjamin Marzinski wrote:
A problem that mpathpersist has with making SCSI Persistent Resevations
to a multipath device work like they do to individual SCSI devices is
that some of the paths to a multipath device might be down or missing
when the mpathpersist commands are run. Multipath handles registering a
new key pretty well. If paths are unavailable at the time of the
command, the key is registered when they later become available. But if
the multipath device is also holding a reservation on one of its paths,
things get trickier.
If a persistent reservation is being held by an unsuable path of a
multipath device (the path can either be down or completely removed),
libmpathpersist can't change it just by forwarding the regular
persistent reservation commands. This can cause problems both for the
RELEASE command and the REGISTER and REGISTER AND IGNORE commands if
they are used to change from one key to another. If the path holding the
reservation is unavailable, the reservation won't be released or have
its key changed, as expected. I wish the problem of having a reservation
key changed while it is holding the reservation was simply a theoretical
one, but there are enterprise users of multipath that need this
capability.
This patchset deals with both of these problems. libmpathpersist always
had code to handle releasing a reservation held by an unavailable path,
but the existing method is broken. It relies on poorly supported
optional features of SCSI Persistent Reservations: the READ FULL STATUS
command and specifying Initiator Ports with the REGISTER command
(SIP_C). Also, fixing its current issues would additionally require
supporting the All Target Ports option (ATP_C). This existing workaround
has been redesigned to use the PREEMPT command instead. Key changes
where the path holding the reservation is unavailable were not
previously handled by libmpathpersist. This patchset also handles them
using the PREEMPT command.
I wish we had a testcase for all of that. Persistent reservation
handling is tricky at the best of times, but throwing in multipathing
it really gets into the arcane knowledge area.
Ben, do you have something which we could turn into some blktest
scenarios?
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
h...@suse.de +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich
