Re: [Xen-users] Disk starvation between DomU's

[Thanos Makatos <thanos.makatos@xxxxxxxxxx>]

> -----Original Message-----
> From: Wiebe Cazemier [mailto:wiebe@xxxxxxxxxxxx]
> Sent: 17 June 2013 16:24
> To: Ian Campbell
> Cc: Thanos Makatos; xen-users@xxxxxxxxxxxxx
> Subject: Re: [Xen-users] Disk starvation between DomU's
> 
> ----- Original Message -----
> > From: "Ian Campbell" <Ian.Campbell@xxxxxxxxxx>
> > To: "Thanos Makatos" <thanos.makatos@xxxxxxxxxx>
> > Cc: "Wiebe Cazemier" <wiebe@xxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxx
> > Sent: Tuesday, 11 June, 2013 12:41:39 PM
> > Subject: Re: [Xen-users] Disk starvation between DomU's
> >
> > ....Or if using blkback the relevant kernel thread.
> >
> > Ian.
> 
> That's what I ended up doing. After first having a certain Domu "best
> effort, 0", I now put it in the real-time class, with prio 3. I can't
> say I notice any 'real-time' performance now. It still hangs
> occasionally.

I'm not sure whether this will work. AFAIK actual I/O is performed by 
tapdisk/qemu, so could you experiment with that instead? Also, keep in mind 
that there is CPU processing in the data path, so have a look at the dom0 CPU 
usage when executing the I/O test.

> 
> Additionally, when I do the following on the virtual machine in
> question:
> 
> dd if=/dev/zero of=dummy bs=1M
> 
> I hardly see any disk activity on the Dom0 with iostat. I see the
> blkback popping up occasionally with a few kb/s, but I would expect
> tens of MB's per second. The file 'dummy' is several GB's big in a
> short while, so it does write.
> 
> Why don't I see the traffic popping up in iostat on the Dom0?

This is inexplicable. Either you've found a bug, or there's something wrong in 
the I/O test. Could you post more details? (E.g. total I/O performed, domU 
memory size, dom0 memory size, average CPU usage, etc.)

What's the array's I/O scheduler? I think since it's a RAID controller the 
"suggested" value is NOOP. If your backend is tapdisk, then CFQ *might* do the 
trick since each domU is served by a different tapdisk process (it may be the 
same with qemu).

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