multipathing pending issues with rhel

[Christophe Varoqui <christophe.varoqui@free.fr>]

Ben,

I'd like to summarize all the issues I raised recently through my
employers support channel on the multipath subsystem.
And see if something can be done about it, at least in the upstream
concerned codebases.

1/ multipathd private namespace pins lvm2 logical volumes maps mounted
at daemon startup, thus making "vgchange -ay" fail, even after
umounting the visible mount. In my context, it also means I can't stop
a clustered service build on this vg to start it on another node. This
problem does not affect upstream which does not create a private
namespace.

2/ can't map a rw multipath over read-only paths. Quick workaround to
create ro multipath, but ro->rw promotion is not automatic when paths
become writable. I keep thinking we should allow rw multipath over ro
paths. The ro->rw event might also work, but what will trigger the
kernel rw status change in the first place ? To my knowledge, only a
manual scsi device rescan can force this status update ... which
accounts for a less user-friendly solution than the former.

3/ Can't use scsi-3 persistent reservations on clariion multipathed
luns : paths reserved on node A, writes submitted on node B should be
errored immediately to ensure data integrity. Instead, writes get
buffered in the "queue_if_no_path" logic, and finally corrupt the data
when reservation get cleared. In my context, reservation is the
prefered io fencing method for clusters.
The kernel knows the write io submitted on a path is refused due to a
reservation conflict, but this status is not propagated to multipath
for it to react by not queuing this io as it should.

Regards,
cvaroqui