Dne 15.5.2015 v 12:45 Atom2 napsal(a):
Hello list,
I am trying to setup a cow snapshot for a LV that is used as a master image
for a number of VMs. The idea basically is to be able to update the master
image even when VMs are up and running; the VMs should then still see the old
state of the image and only when they are restarted they should connect to the
new image.
Searching the net seemed to point towards a snapshot-origin/snapshot solution
- however I am unable to get this to work. Information on the net seems to be
sparse, so I though I'd ask the experts on the list. Here are my steps:
1.) I have a LV in volume group VG named master.ROOT
(/dev/mapper/VG-master.ROOT), 8GB, formatted as ext4
2.) I create a sparese file: truncate -size=8G /tmp/snapshot
3.) losetup -f /tmp/snapshot --> gives /dev/loop0
4.) dmsetup create mytest.img --table "0 $(blockdev --getsz
/dev/mapper/VG-master.ROOT) snapshot-origin /dev/mapper/VG-master.ROOT
5.) dmsetup create mytest.img.cow --table "0 $(blockdev --getsz /dev/loop0)
snapshot /dev/mapper/VG-master.ROOT /dev/loop0 P 8"
So far so good ... however, when I try to mount the origin device by
6.) mount /dev/mapper/mytest.img
the mount call doesn't return and the system gets unresponsive/freezes up to a
point when OOM-killer is being invoked. Login attempts on the console time out
and in essence it is only possible to reboot the system using magic-sysreq key
combinations.
I'd be very much obliged if someone in the know could provide me with
information what's wrong with this approach.
Many thanks in advance Atom2
Do you have any scientific reason to not use LVM2 here ?
Management of snapshot target is not trivial - especially the order
of individual table loads and resumes.
You could look at 'lvcreate -s -vvvv' if you are interested in ioctl ordering
of all operations here.
Regards
Zdenek
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