Re: [Xen-devel] Solution for problems with HyperSCSI and vbds ?

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>]

> Thinking about just 3 DOM0 HyperSCSI clients connecting 
> to the HyperSCSI-Server directly feels somehow more comfortable.
> (e.g. a lot easier administration, less points of failiure.)
> The 3 DOM0s in this example can then export the HyperSCSI
> device(s) via whatever means to the domains > 0.

Of course, the absolutely proper solution is to put HyperSCSI
into Xen, so that Xen's block device interface could be used by
guest OSes to talk directly with the remote disk.

However, I wouldn't want to contemplate putting a big gob of code
like HyperSCSI into Xen until we have implemented the plan for
ring-1 loadable modules support. This would then give us a
shared-memory block device interface between guestOSes and the
HyperSCSI driver (also running in ring1). The HyperSCSI driver
would then talk to the network interface, again using
shared-memory.
 
> Thanks a lot for pointing me to this solution !
> I will look into it during the next days (especially performance ;-).

I'm looking forward to hearing how you get on.

> A propos:
> Did you ever make benchmarks about the average or maximum
> throughput of your VFR implementation in XEN ?

The throughput between domains and the real network interface is
_very_ good, easily able to saturate a 1Gb/s NIC, probably good
for rather more.

However, I'm afraid to say that we recently discovered that our
inter domain performance is pretty abysmal -- worse than our 
performance over the real network, which is simultaneously
amusing and sad. 

The problem is that we currently don't get the asynchronous
`pipelining' when doing inter-domain networking that gives good
performance when going to an external interface: since the
communication is currently synchronous we don't get back pressure
to allow a queue to build up as would happen with a real NIC. The
net result is that we end up bouncing in and out of xen several
times for each packet.

I volunteered to fix this, but I'm afraid I haven't had time as
yet. I'm confident we should end up with really good inter domain
networking performance, using pipelining and page flipping.

> Also, did you make some benchmarks about the amount
> of performance degradation by using vbds/vds for disk access
> compared with using the block device directly (test in DOM0)?

Performance of vbds and raw partitions should be identical. Disks
are slow -- you have to really work at it to cock the performance
up ;-)

> Could mounting /dev/sda via enbd be more performant or
> at least nearly equally performant to using vds and vbds 
> because of the additional overhead of vd/vbd use... ??

Performance using enbd should be pretty good once we've sorted
out inter domain networking.


Ian


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