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

[Ori Mamluk]

On 07/02/2012 14:40, Anthony Liguori wrote:
> On 02/07/2012 06:30 AM, Ori Mamluk wrote:
> > > In addition, I don't see that the listener thread holds any lock while
> > it reads the image. I guess that during that period the guest runs 
> > and may
> > race w/ this new thread.
> >
> > Yes - I mentioned that in the patch mail as one of the open issues. Can
> > you direct me to the lock I need? The function I call from a new thread
> > context is bdrv_aio_readv.
>
> One thing that has me confused about this proposal.  Why not just make 
> rehub be an iSCSI target provider and call it a day?
You've got a point there. This also goes with what Kevin wrote about 
splitting the IOs with a block driver and not at the generic level.

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?
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?
* 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?
* What can you say about NBD via iScsi - with respect to our 
requirements- who is more mature in Qemu?

One more thing about the iScsi initiator - it will not be a standard 
backing for a drive, because the 'production' drive (i.e. the original 
image) is more important than the replicated one. This means that even 
though we use iScsi, this is still a replication agent - not a generic 
'additional' iscsi backing.




> Regards,
>
> Anthony Liguori
>
> > -----Original Message-----
> > From: Dor Laor [mailto:address@hidden
> > Sent: Tuesday, February 07, 2012 2:25 PM
> > To: Anthony Liguori
> > Cc: Ori Mamluk; Kevin Wolf; address@hidden
> > Subject: Re: [Qemu-devel] [RFC PATCH] replication agent module
> >
> > On 02/07/2012 02:12 PM, Anthony Liguori wrote:
> > > Hi,
> > >
> > > On 02/07/2012 04:29 AM, Ori Mamluk wrote:
> > > > Repagent is a new module that allows an external replication system
> > > > to replicate a volume of a Qemu VM.
> > > >
> > > > This RFC patch adds the repagent client module to Qemu.
> > >
> > > Please read http://wiki.qemu.org/Contribute/SubmitAPatch
> > >
> > > In particular, use a tool like git-send-email and split this patch up
> > > into more manageable chunks.
> > >
> > > Is there an Open Source rehub available? As a project policy, adding
> > > external APIs specifically for proprietary software is not something
> > > we're willing to do.
> > >
> > > Regards,
> > >
> > > Anthony Liguori
> >
> > In addition, I don't see that the listener thread holds any lock 
> > while it
> > reads the image. I guess that during that period the guest runs and may
> > race w/ this new thread.
> >
> > About image ID for the replication hub, you can use the VM's pid or VM's
> > uuid paired w/ the specific disk uuid
> >
> > Thanks,
> > Dor