Re: [Qemu-devel] Query regarding qemu dirty bitmap

[John Snow <jsnow@redhat.com>]

Hi Sanjay, usually we reply below or in-line on this list.

On 11/05/2016 11:26 PM, Sanjay Kumar wrote:
> One more question:
>
> 4. How to interpret dirty bitmap file or get list of dirty blocks from
> the dirty bitmap in version 2.5?
>

We still don't have a working export for 2.8, what dirty bitmap file are 
you referencing?

> On Sat, Nov 5, 2016 at 9:29 PM, Sanjay Kumar <isanjaykumar@gmail.com
> <mailto:isanjaykumar@gmail.com>> wrote:
>
>     Hi John,
>
>     Greetings. Hope you have wonderful weekend.
>
>     This is Sanjay Kumar working on a backup and restore solution for KVM.
>
>     The QEMU dirty bitmap QMPs support are awesome.
>
>     I have below queries in regard to that.
>
>     1. How to integrate these QMPs for external backup application.
>              a) Is there any command to get the list of all the dirty
>     blocks?

Not yet. There are plans to extend the NBD protocol to allow a host 
application to query for the status of each block. You would basically 
create an NBD export of a device being tracked and then use the NBD 
protocol to ask for block status.

>              b) We just want list of dirty blocks, how to get those
>     during migration?

Not yet possible. With qcow2 persistence you will have the ability to 
see what was dirty at the time of last flush, but that is not upstream 
yet. With the above NBD extension, you'll be able to see the dirty 
status of any given block at any point in time you want.

>
>     2. How to create just dirty bitmap on a file? I don't need data and
>     want to avoid COW.
>

You can create a dirty bitmap on any type of drive you'd like, but 
persistence is only supported (in patches on-list but not yet merged) 
for .qcow2, with perhaps some support for parallels in the future.

There are no plans to support bitmap persistence on raw files at present.

There have been at times some controversial discussions about allowing 
qcow2 to be backed by other file formats, which would achieve what you 
want -- but there is nobody actively working on this at present -- we're 
more worried about getting the bitmap feature set working first.

>     3. Is there any plan to add query-block-dirty-bitmap command? If
>     yes, then in which version it is coming?
>

If you mean a QMP command that will tell you which blocks are dirty, no. 
We have debated fairly exhaustively and have decided that we do not want 
the QMP command protocol to be used for data transfer.

We may augment existing query commands to provide some extra data in the 
future, perhaps

The solution we arrived at instead was to use the NBD protocol to allow 
clients to query safely the block status, but it requires a protocol 
extension we're still debating on.

>     Please reply. Many thanks in advance.
>
>     With best regards,
>     Sanjay Kumar
>
>

Good luck,
--js