[libvirt-users] Looks like blockpull does not accept a subset of the entire chain of backing files

[Kashyap Chamarthy]

On 01/29/2014 08:21 PM, Richard Gomes wrote:
> Hello
> 
> If I'm not terribly mistaken, looks like libvirt 1.2.1 does not provide
> ability of merging only a subset of the entire chain of backing files.


> 
> So, if I have a chain like this:
> 
>                  root <- a <-b <- c <- d <- active
> 
> ... and  I'd like to obtain a chain like this:
> 
>                   root <-  c <- d <- active
> 
> ... looks like it's not supported, since I'm trying the command
> 
> /                   //virsh blockpull domain --path /path/to/c
> --bandwidth 30 --base /path/to/root//          (1)
> /
> ... but it complains because /--path /path/to/c/ is not recognized,
> since only /--path /path/to/active/ is acceptable:
> 
> /                   //virsh blockpull domain --path /path/to/active
> --bandwidth 30 --base /path/to/root//          (2)
> /
> So, command (1) fails but command (2) succeeds.
> 
> The point is: How could I obtain the results I'm trying to achieve via
> command (1) ?
> 
> I'm new to libvirt, but the article below made me think that what I'm
> trying to do would be possible:
>      
> http://kashyapc.fedorapeople.org/virt/lc-2012/snapshots-handout.html  (
> see section on /blockpull/ )

I still have to make to time to submit a V2 of that that patch[1] with
reflecting latest upstream status.

Apart from what Eric said in painstakingly-detailed way, here's some old
notes from a previous discussion with Eric on mailing lists/IRC. Eric,
please point out if something is wrong below.


- "blockpull" can only pull into the *active* layer.

    - Support to pull into an intermediate/arbitrary layer is in
      progress.

    - NOTES:

       - Pulling into intermediate images requires opening the
         intermediate file read/write, as well as ensuring that any
         reads done in the active layer get correct data.

       - Case of 'reading data' - In the chain base <- snap1 <- active
         if you pull base into snap1, reading active has to know whether
         the data comes from base or from snap1 . But if you pull into
         active, you already have the destination read/write, and once
         the data is pulled, you don't have to worry about keeping the
         chain consistent.

       - Case of 'writing data' -- If you pull into the active layer,
         then a write eliminates the need to pull that cluster from the
         backing file, thanks to copy-on-write semantics. But, if you
         pull into an intermediate layer, a write at the active layer
         doesn't affect the fac t that you still have to pull into the
         intermediate layer. Even though the active layer won't use that
         cluster from the intermediate layer.

- At the moment, virsh blockpull cannot be done 'offline' (although,
  'qemu-img' can do that.)


[1] http://www.redhat.com/archives/libvir-list/2013-January/msg01903.html


-- 
/kashyap