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

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/07/2012 08:40 AM, Paolo Bonzini wrote:
> On 02/07/2012 03:06 PM, Ori Mamluk wrote:
>> The main issue about it is that the Rephub also needs the other
>> direction - to read the protected volume. I get the feeling that with
>> live block copy and NBD there's probably something that might fit
>> this need, no?
>
> Yes, with two NBD sockets you could do it. But would you use both at the same
> time? I would have thought that either the rephub is streaming from the
> protected volume, or QEMU is streaming to the rephub.
>
> The current streaming code in QEMU only deals with the former. Streaming to a
> remote server would not be supported.
>
>> With a 'new' agent like I need this is relatively easily achieved by a
>> bidirectional protocol, but I agree a more generic protocol would be
>> more elegant, although it will probably require a socket per direction, no?
>>
>> I Some smaller questions:
>> * Is there already a working iScsi initiator as a block driver (I hope
>> I'm using the right terminology) in Qemu, or do I need to write one?
>
> Yes, there is one using libiscsi. But I think Anthony was not referring to iSCSI
> in particular, NBD would work just as well.
>
>> * This driver would need to be added in run-time - to allow starting to
>> protect a running VM. Maybe via a monitor command. I guess that's OK,
>> right?
>
> Yes, I think you can detach a block device from a drive and reattach the new
> mirroring device.
>
>> * What can you say about NBD via iScsi - with respect to our
>> requirements- who is more mature in Qemu?
>
> Personally I prefer NBD because it is lighter-weight and there is a server
> inside QEMU (so you can use it easily with non-raw images). It is more mature,
> but it is a bit less extensible.

Which is also fine.

You could also just use DRBD ;-)

Regards,

Anthony Liguori