Snapshot merge for the device mapper

Posted Nov 26, 2009 3:18 UTC (Thu) by intgr (subscriber, #39733)
Parent article: Snapshot merge for the device mapper

So using the broken upgrade example... How does DM tell the file system that "your underlying block device has been changed arbitrarily, please discard everything you know"?

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Snapshot merge for the device mapper

Posted Nov 26, 2009 4:33 UTC (Thu) by msnitzer (guest, #57232) [Link] (4 responses)

DM does not change the block device out from underneath a mounted filesystem. The filesystem (and processes that are using it) would not be able to reliably cope with such an abrupt change to its underlying block device.

The user must decide to merge a snapshot. Once that decision is made the user will initiate the merge using lvm2's lvconvert --merge $VG/$LV

But the actual merge cannot start until both the snapshot that will be merged and its origin device are closed.

So if either the snapshot or origin device are still open (e.g. the filesystem ontop is still mounted) the merge is deferred until the origin LV is next activated.

Snapshot merge for the device mapper

Posted Nov 26, 2009 6:10 UTC (Thu) by intgr (subscriber, #39733) [Link] (1 responses)

Well the article suggests that the new feature can "be used to roll back an
unfortunate upgrade without taking the system down"

But needing to unmount the root file system seems pretty much like "taking
the system down" to me. In other words, nothing fundamentally changes, it
just got more convenient.

Snapshot merge for the device mapper

Posted Nov 26, 2009 12:10 UTC (Thu) by nye (subscriber, #51576) [Link]

That's a fairly major improvement in convenience though. I for one would welcome this very warmly after my LVM escapades a couple of weeks ago.

Snapshot merge for the device mapper

Posted Dec 2, 2009 17:29 UTC (Wed) by sethml (guest, #8471) [Link]

I think the ideal behaviour would be if you could swap a snapshot with the HEAD state of the partition, effective on the next unmount-mount cycle. Then restoring the system to a snapshot state would consist of swapping that snapshot into place and rebooting. You'd still have the borked former HEAD state of the system in a snapshot, which you could delete at will. You're not likely to get more convenient than that.

Snapshot merge for the device mapper

Posted Feb 25, 2010 15:28 UTC (Thu) by budee (guest, #63877) [Link]

According to ftp://sources.redhat.com/pub/lvm2/WHATS_NEW you must have lvm version at least 2.02.58 to use --merge.