libvirt users - Apr 2019 - Live Disk Backup with Active Blockcommit

Hi!

I was reading the Wiki about 'Live Disk Backup with Active Blockcommit,
https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit
and see that example shown seems to be using two different names for the same
file.

First showing the current block device in use:
$ virsh domblklist vm1
Target     Source
------------------------------------------------
vda        /export/images/base.qcow2 

Then copying the same file for backup:
$ cp /export/images/base.img /export/images/copy.img

=> Is there are reason why the block device first is called
'base.qcow2' and then 'base.img'?


I also saw another name that seems to be wrong based on the context.
It doesn't have the same implication as my earlier example, as the name of
the overlay file is not written in the command itself, but still:
------
$ virsh snapshot-create-as --domain vm1 guest-state1 \
    --diskspec vda,file=/export/images/overlay1.qcow2 \
    --disk-only --atomic

Then:
$ virsh domblklist vm1
Target     Source
------------------------------------------------
vda        /export/images/overlay.qcow2

Now that the backup is finished, perform active blockcommit by live mergning
contents of 'overlay1' into base . . . ."
------

=> Here it seems like the overlay file first is called
''overlay1.qcow2" and the ' virsh domblklist vm1' command
shows that it is actually called: 'overlay.qcow2'
And then back to ''overlay1 in the following text: 'mergning
contents of 'overlay1' into base'
(Where 'mergning' is spelled wrong, but that doesn't affect the
logic).


=> Do I understand all this correctly?

I came to this article based on this answer on Serverfault:
https://serverfault.com/a/921966
Since I didn't understand the logic of the file changing names, I visited
the file linked to in the answer on wiki.libvirt.org and saw that the same was
written there.
So maybe change these errors in the Wiki if I understand all this correctly?


Thanks for input!

Best regards,
Wolf

On 4/25/19 8:59 AM, Wolf wrote:
> Hi!
> 
> I was reading the Wiki about 'Live Disk Backup with Active Blockcommit,
https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit
> and see that example shown seems to be using two different names for the
same file.
> 
> First showing the current block device in use:
> $ virsh domblklist vm1
> Target     Source
> ------------------------------------------------
> vda        /export/images/base.qcow2 
> 
> Then copying the same file for backup:
> $ cp /export/images/base.img /export/images/copy.img
> 
> => Is there are reason why the block device first is called
'base.qcow2' and then 'base.img'?
Nope. Fixed to favor .img throughout, to emphasize that the base image
can be ANY format, not just qcow2.
> 
> 
> I also saw another name that seems to be wrong based on the context.
> It doesn't have the same implication as my earlier example, as the name
of the overlay file is not written in the command itself, but still:
> ------
> $ virsh snapshot-create-as --domain vm1 guest-state1 \
>     --diskspec vda,file=/export/images/overlay1.qcow2 \
>     --disk-only --atomic
> 
> Then:
> $ virsh domblklist vm1
> Target     Source
> ------------------------------------------------
> vda        /export/images/overlay.qcow2
> 
> Now that the backup is finished, perform active blockcommit by live
mergning contents of 'overlay1' into base . . . ."
> ------
> 
> => Here it seems like the overlay file first is called
''overlay1.qcow2" and the ' virsh domblklist vm1' command
shows that it is actually called: 'overlay.qcow2'
> And then back to ''overlay1 in the following text: 'mergning
contents of 'overlay1' into base'
> (Where 'mergning' is spelled wrong, but that doesn't affect the
logic).
Also fixed, here preferring overlay1.qcow2 since it is possible to
create a chain of more than one overlay.
> 
> 
> => Do I understand all this correctly?
Yes, thanks for spotting the issues and reporting them.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org