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[lvm-devel] Re: Deactivating volume with mounted snapshot

On Thu, 15 May 2008, Alasdair G Kergon wrote:

On Wed, May 14, 2008 at 08:33:27PM -0400, Mikulas Patocka wrote:
Basically, lvm suspends all the devices, then loads the dm table for all
of them and then resumes them.

Not strictly accurate.

You need to consider each case separately:

 taking existing LV and adding/removing first snapshot
 adding/removing a subsequent snapshot
 activating/deactivating LV plus pre-existing snapshots

and note the sequence of significant events in each case:
 table loaded
 snapshot metadata read from disk
 filesystem quiesced
 snapshot becoming active

Then you'd need to show that removing 'active = 1' retains correctness
in every case.  I am not convinced: Consider the case of adding a
subsequent snapshot: preload table does mem allocations, then suspend
existing LV & quiesce fs, then resume snapshot (active = 1 now) etc.

agk redhat com

I put a printk points at ctr/dtr/suspend/resume functions into origin and snapshot and observed their calls when activating/deactivating, adding/removing a snapshot and adding/removing the first/last snapshot.

Because "active" variable was never reset to zero, it had effect only between snapshot ctr (set to 0) and snapshot resume (set to 1). When it was set to 0, the snapshot didn't allocate exceptions. After the patch, the snapshot always allocates exceptions (between snapshot ctr and snapshot dtr calls).

The change in the patch is safe because when someone is creating snapshot and simultaneously writing to the origin device, it is unspecified, which write requests trigger reallocations in the snapshot and which not.

I found another problem --- the lvchange -an is currently safe, but it is unsafe with snapshot merging. Ideally it should suspend all the devices first and then call destructors on the snapshots. I'll look at it.


lvchange -ay:
origin ctr: 254:4
snapshot ctr 254:4 254:6
snapshot ctr 254:4 254:8
origin resume: 254:4 254:6
origin resume: 254:4 254:8
origin resume: 254:4

lvchange -an:
origin suspend: 254:4
origin dtr: 254:4
snapshot suspend: 254:4 254:6
snapshot dtr 254:4 254:6
snapshot suspend: 254:4 254:8
snapshot dtr 254:4 254:8

lvcreate snapshot:
snapshot ctr 254:4 254:11
origin suspend: 254:4
snapshot suspend: 254:4 254:6
snapshot suspend: 254:4 254:8
snapshot resume: 254:4 254:6
snapshot resume: 254:4 254:8
snapshot resume: 254:4 254:11
origin resume: 254:4

lvremove snapshot:
snapshot suspend: 254:4 254:11
snapshot dtr 254:4 254:11

lvcreate, adding the first snapshot:
origin ctr: 254:12
snapshot ctr 254:12 254:13
snapshot resume: 254:12 254:13
origin resume: 254:12

lvremove, removing the last snapshot:
snapshot suspend: 254:12 254:13
snapshot dtr 254:12 254:13
origin suspend: 254:12
origin dtr: 254:12

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