[linux-lvm] Snapshots and disk re-use
James Hawtin
oolon at ankh.org
Wed Apr 6 01:36:40 UTC 2011
On 06/04/2011 00:43, Stuart D. Gathman wrote:
> In fact, since you still have to zero an LV before removing, you might
> as well
> zero an LV when you allocate. Same overhead. Then you don't need to
> mess
> with any of these schemes to deal with snapshots. Did we already cover
> that obvious solution? We did discuss a utility to optimize zeroing an
> already mostly zero LV.
>
I think you mean and lv of a virtual machine rather than snap shot, not
sure how you could zero the cow. Stuart does have a good point here,
there is another source of leak outside of snapshots (and probably
worse), if you delete an lv the create a new one over the same PEs the
data from then old lv will be there, save a few k that is zeroed at the
front (to help with fs/label creation). This is worse as it is in order
data that the cow from a snap is not. Geting a while fs back is very
possible.
Try this test
lvcreate -L 256m -n test_lv osiris_l1_vg
yes | dd of=/dev/osiris_l1_vg/test_lv
pvdisplay --map
lvremove /dev/osiris_l1_vg/test_lv
lvcreate -L 256m -n test_lv osiris_l1_vg
pvdisplay --map
od -ha /dev/osiris_l1_vg/test_lv
James
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