Re: [Qemu-devel] [RFC PATCH] replication agent module

[Paolo Bonzini]

On 02/08/2012 01:03 PM, Stefan Hajnoczi wrote:
> If you intend to run an unmodified drbd server on the rephub, then it
> may not be possible to get point-in-time backups.  (Although this
> probably depends since things like btrfs or zfs may allow you to get
> back to arbitrary transactions or timestamps.)

I'm not sure what's the overhead, but btrfs copy-on-write (reflinks) may 
help.

> But you could consider drbd as a network protocol and implement your
> own server which speaks the protocol.  Then you can add any
> functionality you like, just like the case with the proprietary rephub
> server you mentioned in your patch.
>
> So the only difference is that instead of using a new custom protocol
> the rephub would need to speak the drbd protocol.

So you're suggesting DRBD-over-NBD on the client, and for the 
replication hub a custom server speaking the DRBD protocol?  I didn't 
find any documentation for DRBD and the code is only in the kernel, so 
this sounds like a lot of work.

What about taking the existing Ceph/RBD driver in QEMU and changing it 
to support arbitrary image formats rather than just raw?  That sounds 
much much easier.  The main advantage is that Ceph has a user-space 
library for use in the replication hub.  It also supports snapshots.

Paolo