Re: [Qemu-devel] Live Block Migration using Mirroring

[Federico Simoncelli]

----- Original Message -----
> From: "Paolo Bonzini" <address@hidden>
> To: address@hidden
> Sent: Tuesday, February 28, 2012 7:02:40 PM
> Subject: Re: [Qemu-devel] Live Block Migration using Mirroring
> 
> Il 28/02/2012 18:46, Federico Simoncelli ha scritto:
> > > > Thank you for getting this. Being able to have a bogus backing
> > > > file
> > > > was a bonus but it's not really required for the mirrored live
> > > > block
> > > > migration. We can add the support for switching the backing
> > > > file in the
> > > > drive-reopen part.
> > > 
> > > Wait, it's not really required for oVirt because it creates the
> > > snapshot outside QEMU.  What about everyone else?
> > 
> > They'll have (as oVirt) two mirrored snapshot pointing at the same
> > base. The only difference is that the image is created internally,
> > but
> > that's not hard.
> 
> Can you detail how you are switching the backing file in the
> drive-reopen?  Either the BlockDriverState opened by blkmirror, or
> the
> one opened at the end, will have to use the "wrong" backing_file.
>  How
> do you arrange for that?

Step 1 - Initital Scenario
==========================
VM1 is running on the src/hd0base.

[src/hd0base] <= VM1(read-write)

Step 3 - Mirrored Live Snapshot
===============================
A mirrored live snapshot is issued using src/hd0snap1 and dst/hd0snap1 as image
files (both having src/hd0base as backing file).

[src/hd0base] <- [src/hd0snap1] <= VM1(read-write)
             ^-- [dst/hd0snap1] <= VM1(read-write)

Step 4 - Backing File Copy
==========================
An external manager copies src/hd0base to the destination (dst/hd0base).

[src/hd0base] <- [src/hd0snap1] <= VM1(read-write)
[dst/hd0base]^-- [dst/hd0snap1] <= VM1(read-write)


Step 5 - Final Switch to Destination
====================================
VM1 is now able to switch to the destination for both read and write operations
fixing the backing file path in dst/hd0snap1.

[src/hd0base] <- [src/hd0snap1]
[dst/hd0base] <- [dst/hd0snap1] <= VM1(read-write)

-- 
Federico