Am 15.05.15 um 19:58 schrieb Zdenek Kabelac:
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 ??
Well I simply haven't found anything within LVM2 that supports my use
case. Everything I have found is about buffering writes to the original
device such that the original device appears unchanged.
I am, however, curious as well: What makes you think that my use case is
covered by pure LVM2? Do you have a link with the steps required - that
would be great!
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.
I have seen that, but I got the impression, that's the other way round
to what I require: It creates a snapshot that copies writes to the
original device in order for the original volume to be in a consitent
state e.g. for bacups; those writes will then later be merged back to
the original volume. That's, however, unfortunately not what I require.
What I require is that upon a write to the original device the _old_
block is copied to the snapshot cow device so that any process accessing
the snapshot has a consistent (and unchanged) view of the original
device. The original device itself will change!
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/
It looks as if I am not smart enough and (at least) in the area of
device-manager (which is expected) you are much smarter than me. So let
me pick your brain: Would you be so kind to outline the steps required
for me to be able to setup my use case with LVM2: Again, I need a
snapshot device that only stores changed blocks from the snapshot-origin
device (which is used somewhere else) and routes unchanged read requests
back to the origin device. I am still not sure whether we are on the
same page here.
I am also very curious to understand why the exact same steps outlined
further above _do_ work when both the snapshot-origin device and the
snapshot device are based on losetup (spares-)file based block devices
and fail when the snapshot-origin device is a block device created by
LVM2 and only the snapshot device is a losetup (sparse-)file based block
device.
Many thanks again for your patience, but I'd very much appreciate if you
could be more explicit in your (at least for me) rather vague answers. I
am also happy to join a session on IRC - but that channel seems to be
pretty orphand at all times.
Thanks Atom2
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