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

[Mike Snitzer]

On Wed, Sep 21 2016 at  5:14am -0400,
jiangyiwen <jiangyiwen at huawei.com> wrote:

> 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!

Which kernel are you using?  Christoph (now cc'd) has introduced a new
PR api.  I merged a fix from him relatively recently (commit 9c72bad1f3
"dm: call PR reserve/unreserve on each underlying device")

Mike