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Description of problem:

This value hangs around in a device's udev properties even when device is no longer multipathed. Other DM_MULTIPATH_* values disappear.

Version-Release number of selected component (if applicable):

Name        : device-mapper-multipath
Arch        : x86_64
Version     : 0.4.9
Release     : 85.el7
Size        : 178 k
Repo        : installed
From repo   : RHEL-7

How reproducible:

Seems consistent.

Steps to Reproduce:
1. Remove some multipath devices from a group.
2. Use udevadm to list their udev properties.
3. Notice that DM_MULTIPATH_TIMESTAMP remains even while other DM_MULTIPATH_* properties are gone.

Actual results:

DM_MULTIPATH_TIMESTAMP remains, other DM_MULTIPATH_* properties gone.

Expected results:

I was a little surprised to see any DM_MULTIPATH_* properties remaining for the device.

Additional info:

This may be quite deliberate, but even a reboot doesn't seem to clear it.

DM_MULTIPATH_TIMESTAMP will always be there when multipathing is enabled and multipathd is running.  It's purpose it to cut down on how often multipath has to check if a device is a multipath path.  This can take a bit of time, so we don't want to check on every change event.  Instead, multipathd updates a timestamp whenever a new path device is discovered for the first time, or the configuration file is reloaded.  When a change event happens, the device checks its existing timestamp value against the current one, and if it has changed, multipath will recheck the path.