Re: [Qemu-devel] [PATCH] This patch adds a new block driver : iSCSI

[Orit Wasserman]

On Thu, 2011-09-15 at 13:58 +0200, Paolo Bonzini wrote:
> On 09/15/2011 01:42 PM, Dor Laor wrote:
> >>
> >> However, iSCSI is a complex protocol and a complex suite of tools. With
> >> a simpler set of tools such as qemu-nbd/nbd, you can for example tunnel
> >> data over the libvirt connection. Possibly with encryption. Also, with
> >> iSCSI you're tied to raw, while qemu-nbd lets you use qcow2.
> >
> > My main motivation for external iScsi is to provide qemu operations w/
> > non shared storage. Things like streaming an image w/o shared storage or
> > block live migration can be done where the src host exposes iscsi target
> > and the destination is the initiator. In case of qcow2, every snapshot
> > in the chain should be exposed as a separate LUN. The idea is to ignore
> > the data in the guest image and treat it as opaque.
> 
> Then you need an iSCSI *target* that understands qcow2, like qemu-nbd 
> but for iSCSI... that's exactly the thing you were worried about 
> implementing.
Not really , this is just part of the target configuration.
For each file in the chain we create a lun in the target that is backed
by the file and we number the luns in chronological order (base is lun
1) . you can look at a tgtd configuration here
http://wiki.qemu.org/Features/LiveBlockMigration#Example_single_base_master_image.
Orit

> 
> (Of course you could use qemu-nbd and expose /dev/nbd* via iSCSI, but it 
> starts looking like an exercise in adding layers of indirections! :)).
> 
> Paolo
>