[linux-lvm] Single snapshots do weird things?

Glen Harris glen at gbrmpa.gov.au
Tue Sep 16 17:26:01 UTC 2003

Alasdair G Kergon wrote:
 > On Tue, Sep 16, 2003 at 09:12:15AM +1000, Glen Harris wrote:
 >>Is it mandatory to sync the filesystem before taking a snapshot?
 > If you want the snapshot to be consistent: that's what the VFS lock patch
 > attempts to do automatically whenever you take a snapshot: presumably you
 > haven't applied that patch?
 > Alasdair

Yes, I applied linux-2.4.21-VFS-lock.patch as well as the devmapper patch.
The behaviour is reproducable, I just did it again:

lvm:~# mount /dev/vg0/lv0 fs
[Several files/dirs exist, but not device-mapper.1.00.05.tgz]

lvm:~# lvcreate -s -L 8m --n snap0 /dev/vg0/lv0
lvm:~# mount /dev/vg0/snap0 snap/0
[Not there yet!]

lvm:~# cp /usr/src/device-mapper.1.00.05.tgz fs
lvm:~# lvcreate -s -L 8m --n snap1 /dev/vg0/lv0
lvm:~# mount /dev/vg0/snap1 snap/1
[Does *not* exist in the mounted snapshot filesystem]

lvm:~# sync
lvm:~# lvcreate -s -L 8m --n snap2 /dev/vg0/lv0
lvm:~# mount /dev/vg0/snap2 snap/2
[Ahhh! Now it exists in *this* mounted snapshot filesystem!]

Oh, and after the file appears in the final snapshot, it doesn't magically
appear in the previous one.


| Nine megs for the secretaries fair,      | One disk to rule them all,     |
| Seven megs for the hackers scarce,       | One disk to bind them,         |
| Five megs for the grads in smoky lairs,  | One disk to hold the files     |
| Three megs for system source;            | And in the darkness grind 'em. |
| Glen Harris | glen at gbrmpa.gov.au | 07 47500893 | http://www.gbrmpa.gov.au |

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