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

[Paolo Bonzini]

On 09/15/2011 10:48 AM, Dor Laor wrote:
> > We need the same patch for NBD, so I wouldn't bother with the
> > synchronous flush.
>
> It seems to me that using a qemu external initiator/target pairs like
> Orit's original design in
> http://wiki.qemu.org/Features/LiveBlockMigration#ISCSI_for_non_shared_storage
> would be a simpler (in terms of qemu code..) and flexible solution.

Yes, it would be simpler for QEMU because everything is done outside.

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.

iSCSI could be a better choice if everything in the QEMU block layer was 
done in terms of SCSI.  However, we're a far cry from that.

> I agree that embedding the iscsi initiation in qemu can simplify the end
> user life but since this scenario is expected to be used by higher level
> software it's not relevant here. The risk is to have to maintain more
> code that won't be as general as the tgtd/lio solutions out there.

I'm not sure I understand.  You say "embedding the iSCSI initiator in 
qemu can simplify the end user life" but "the risk is to have to 
maintain [another iSCSI target]".  I don't think anybody proposed adding 
an iSCSI target to QEMU (in fact tcm_vhost would even let you use lio 
instead of QEMU's SCSI target code!).  Only an iSCSI initiator which is 
not much code because it's just a wrapper for libiscsi.

In general, I'm not sure that libiscsi (instead of the in-kernel iSCSI 
initiator) is by definition a bad choice in high-end set-ups.  If you 
want to do SCSI passthrough, it's likely better to use libiscsi rather 
than using the in-kernel initiator together with scsi-generic 
(scsi-generic sucks; bsg is better but also a useless level of 
indirection IMO).

Perhaps Ronnie has rough performance numbers comparing in-kernel iSCSI 
with libiscsi?

Paolo