Dne 15.5.2015 v 18:47 Atom2 napsal(a):
Am 15.05.15 um 14:11 schrieb Zdenek Kabelac:
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 ?
As far as I understand the snapshot-origin target with a cow snapshot is not
(yet) directly supported by LVM2.
Just curious - from where have you got this idea ??
Therefore I have so far worked from an assumption that I need to use dmsetup
directly to solve my use case. If, however, you tell me otherwise and are able
to show me how to use LVM2 instead, I am more than happy to go down that
route. Let me once again outline my use case:
I have a LV (that's the master image/template which was actually setup with
LVM2) which is maintained from within a template VM and serves as the (r/o)
root image for a number of other (dependent) virtual machines. All dependent
virtual machines mount that template LV r/o and overlay it with an overlayfs
as a r/w layer for write access.
I now want to be able to update the master image from within the template VM
and be able to update it at any time; running dependent VMs should however not
see any changes to the image while they are up, hence the requirement to have
a snapshot-origin target and a snapshot that would cow buffer any changes in
the template (origin) target until such time the dependent VM is restarted. At
that point in time the dependent VM would (if required setup and) connect to a
new snapshot.
I hope that clarifies my setup.
Furthermore, IMHO and in any case mounting a dm-target should under no
circumstances in essence bring down a machine - even if something is horribly
wrong.
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.
I am probably missing something here, but my steps listed above do not involve
lvcreate ... so how could I use that -vvv?
Well even kernel documentation for the snapshot target itself is rather
referencing usage of snapshot via lvm2 - and it's for a reason.
https://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt
So if you could use lvm2 - stay with lvm2 and avoid your home-brew dmsetup
commands - it needs deep understanding how the old snapshot target works:
http://people.redhat.com/agk/talks/LVM2-LinuxTag2006/
Regards
Zdenek
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