Multiple snapshots do weird things?

From a few notes here and there, I've been under the impression that multiple snapshots of a single LV are possible. The problem is that whenever I try to take more than two, everything falls apart.

I'm running Debian unstable, kernel 2.4.20, with the following packages:
kernel-patch-device-mapper 1.00.03-1
libdevmapper1.0 1.00.03-1
lvm2 2.00.06-1
lvm-common 1.5.9.3

I've also applied the linux-2.4.20-VFS-lock.patch from the device-mapper 1.00.05 tarball at sistina.

The kernel compiles fine and dm-mod loads. I've run devmap_mknod.sh from the tarball as well, and I can create PVs, VGs and LVs. I can make file- systems on them which survive reboots, and everything looks fine.

I have a shell script which creates a snapshot named for the current time:
===
#!/bin/bash
WHEN=date "+%Y%m%d%H%M%S"
lvcreate -L 4M -s -n $WHEN /dev/vg0/lv0
mkdir /test/snap/$WHEN
mount /dev/vg0/$WHEN /test/snap/$WHEN
===

So, here's an example session which demonstrates my problem. Everything looks fine until the third snapshot, which gives an I/O error when I try to mount it, and also appears to trash the first snapshot - "ls -la" doesn't even list "." or ".."!

test:~# pvcreate /dev/hdb
  Physical volume "/dev/hdb" successfully created

test:~# vgcreate -s 256k vg0 /dev/hdb
  Volume group "vg0" successfully created

test:~# lvcreate -L 512m -n lv0 vg0
  Logical volume "lv0" created

test:~# mke2fs /dev/vg0/lv0
[SNIPPED - Believe me, it worked fine]

test:~# mount /dev/vg0/lv0 /test/fs

test:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda2               841340    742420     56184  93% /
/dev/mapper/vg0-lv0     507748        13    481521   1% /test/fs

test:~# ./snap
  Logical volume "20030910123716" created

test:~# cp -R * /test/fs/

test:~# sync

test:~# ./snap
  Logical volume "20030910123756" created

test:~# rm -rf /test/fs/src/

test:~# ./snap
  Logical volume "20030910123812" created
/dev/vg0/20030910123812: Input/output error
mount: you must specify the filesystem type

test:~# ls -la /test/snap/20030910123716/
total 0

test:~# ls -la /test/snap/20030910123756/
total 22
drwxr-xr-x    4 root     root         1024 Sep 10 12:37 .
drwxr-xr-x    5 root     root         4096 Sep 10 12:38 ..
drwx------    2 root     root        12288 Sep 10 12:37 lost+found
-rwxr-xr-x    1 root     root           62 Sep 10 12:37 makelv0
-rwxr-xr-x    1 root     root           41 Sep 10 12:37 makelv0~
-rwxr-xr-x    1 root     root          166 Sep 10 12:37 snap
-rwxr-xr-x    1 root     root          151 Sep 10 12:37 snap~
drwxr-xr-x    5 root     root         1024 Sep 10 12:37 src

test:~# ls -la /test/snap/20030910123812/
total 8
drwxr-xr-x    2 root     root         4096 Sep 10 12:38 .
drwxr-xr-x    5 root     root         4096 Sep 10 12:38 ..

Thanks, glen.

| 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 | [email protected] | 07 47500893 | http://www.gbrmpa.gov.au |
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Firstly, can you post the output of 'lvm version' and confirm that your device-mapper driver version is version 4.0.4 ?
e.g. 4.0.4-ioctl (2003-08-30)

Alasdair

[email protected]