I tried to do this manually with the latest rhel-6.6 kernel, and the dmsetup patches, and it things got hung up pretty badly.  It's possible that there needs to be some kernel work to make this work correctly on multipath targets.  I'm pushing this out the 6.7 and I'll take a deeper look into it in a little bit.

Created attachment 908622 [details]
Log of unmounting a multipath device with no paths, set for deferred remove

This is a log from the serial console, when I finally unmount a multipath device set to deferred remove.  I'm did this over the serial console, because once this is finished.  all the ssh logins to the node hang.  Eventually the serial console login did to.

You should note that I ran this test with a multipath device that has no paths.  The reason for this is because I want to use deferred removes for exactly this case.  When the last path of a multipath device is removed, multipathd will try to delete it.  However, if it is open, the device can't be deleted.  Unfortunately, this means that the device just sits around, even once the last opener finally closes it.  I was planning on dealing with this cleanup in multipathd, but when Mikulas posted the deferred remove code, it seemed like an ideal way to do it.  I was just verifying that I could do this manually through dmsetup before adding the code to multipathd, when I hit this issue.

I'm going to verify that I don't see the same issue with a linear device or a multipath device with active paths, but I assume that it's specific to the way I want to use it for auto-removing devices with no paths and no users.

Trying this with a linear device worked fine.  Trying this with a multipath device that still had valid paths failed in the same way as a multipath device with no paths.

kernel patch available on bug 1109431 fixes this problem

*** This bug has been marked as a duplicate of bug 1109431 ***