[dm-devel] snapshot-origin freezes system - what am I doing wrong?

[Atom2]

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.

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?

Thanks, Atom2