"create snapshots of any block device"

"create snapshots of any block device"

[Hauke Laging]

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Hello,

I am a bit surprised to read this:
"Device-mapper allows you, without massive data copying:

*) To create snapshots of any block device [...]"

here:
http://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt

Isn't that at least misleading? Is it really possible to create a snapshot of 
e.g. /dev/sda5 without creating an *additional* device as snapshot origin?


I export LVs to a VM where they appear as regular disks so that I cannot use  
LVMs snapshot integration. Thus I create a dm device with a linear mapping to 
each of these disks and mount the dm device, not the disk.

I just had the idea to create these dm layer devices as snapshot-origins from 
the start (instead of loading this target when a snapshot is really going to 
be created). Is there anything to be said against this? Is there any reason at 
all to use linear instead of snapshot-origin for devices with just one target 
(except for possible confusion)?


Hauke
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Re: "create snapshots of any block device"

[Bryn M. Reeves]

On 01/22/2013 11:12 AM, Hauke Laging wrote:
> Isn't that at least misleading? Is it really possible to create a snapshot of
> e.g. /dev/sda5 without creating an *additional* device as snapshot origin?

It's possible but as you note you have to create the dm layers for the 
snapshot. This means that if a non-dm device is in use you have to shut 
its users down (unmount file systems etc.), create the snapshot and then 
re-mount.

I've frequently used this in the past to snapshot large zero targets to 
create sparse devices for testing (before --virtualsize was added to LVM2).

With device-mapper devices this is unnecessary since the top-level 
device may be suspended while the snapshot layers are inserted.

Regards,
Bryn.