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

[Stefan Hajnoczi]

On Wed, Feb 8, 2012 at 6:10 AM, Ori Mamluk <address@hidden> wrote:
> On 07/02/2012 17:47, Paolo Bonzini wrote:
>> But I'm also starting to wonder whether it would be simpler to use
>> existing replication code.  DRBD is more feature-rich, and you can use it
>> over loopback or NBD devices (respectively raw and non-raw), and also store
>> the replication metadata on a file using the loopback device. Ceph even has
>> a userspace library and support within QEMU.
>>
> I think there are two immediate problems that drbd poses:
> 1. Our replication is not a simple mirror - it maintains history. I.e. you
> can recover to any point in time in the last X hours (usually 24) at a
> granularity of about 5 seconds.
> To be able to do that and keep the replica consistent we need to be notified
> for each IO.

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.)

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.

Stefan