dm-mpath: Handling SCSI-3 PR RELEASE in multi-controller environment

[jiangyiwen <jiangyiwen@huawei.com>]

Hi guys,

I'm sorry if someone else has already asked the same question before, but
here's what we are facing with generic multipath.

We were evaluating SCSI-3 PR and hit the following issue:

Our IPSAN has two controllers (C1/C2). Our host is connected to both
controllers. So we have 2 I_T nexus. The same key is registered on each I_T
nexus. When we issue SCSI-3 PR RESERE through generic multipath, the
reservation will be placed on one of the controllers (say, C1). Now if we
issue SCSI-3 PR RELEASE, generic multipath will select _only_ one of its
paths based on user defined path selection policy. The problem is that, if
the selected path leads to controller C2, which does not hold the
reservation, the reservation won't be released - and what's more, since the
reservation is done by the same host, the RELEASE command returns
successfully. So this gives us the illusion that the reservation is released
but in fact it is still there.

I know that there's a user space tool mpathpersist that can handle this
problem by issuing release command on all available paths, but here I am
talking about doing same thing in kernel space. So my question is, is there
anyone who is working on supporting SCSI-3 PR in a multiple controller
environment?

Please advise. Thanks in advance!

Best regards,
Yiwen