RE: Subject: RE: [Xen-users] Poor disk io performance in domUs

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> David Brown
> Sent: 22 June 2007 16:16
> To: Andrej Radonic
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: Subject: RE: [Xen-users] Poor disk io 
> performance in domUs
> 
> On 6/22/07, Andrej Radonic <rado@xxxxxxxxxxxxx> wrote:
> > Mats,
> >
> > >> dd simultaneously in both dom0 = 170 MB/s
> > > I take it you mean "two parallel 'dd' commands at the 
> same time"? That
> > > would still write to the same portion of disk (unless you 
> specifically
> > > choose different partitions?)
> >
> > it's different partitions - one dedicated partition for 
> each domU. The
> > partition are created as "virtual" block devices with the 
> dell storage
> > box manager.
> >
> > >> dd simultaneously in two domU = 34 MB/s
> > > I take it this means two different DomU doing "dd"?
> > > Is that 34 MB/s "total" (i.e. 17MB/s per domain) or per 
> domain (68 MB/s
> > > total)?
> >
> > sorry, good you asked: it's the total, i.e. 17MB/s per 
> domain! I guess
> > you are getting the picture now as to my feelings... ;-)
> >
> 
> Yeah I've experienced some interesting things with very good I/O
> performance and xen not handling it very well with the domU's. Since
> there's a little kernel process running on the dom0 for each virtual
> block device exported to domU's, which does translation mostly, I've
> found that the more domU's you bring up all doing I/O the dom0
> processes tend to do just as much work as all the dd operations of all
> the domU's combined. So if you have 6 domU's all doing about 15% using
> dd's your dom0 is going to be pushing 100% of its cpu usage and going
> to be doing a crap load of work and the I/O performance in the domU's
> will be failing. So it does pay to make sure your dom0 can handle
> translating everything (note this should go away with the IOMMU
> support, I would hope).

Actually, if you expect IOMMU to solve the problem, you can do the same
(with slightly less security, admittedly) in Para-virtual domains today
- since IOMMU can only translate and protect on a per-device level, so
you need to have one device per domain. 

So if you have a disk-controller with disk for each domain, you could do
that today. Same with network controllers [there are even some network
controllers which are "multihead", meaning that they present themselves
as multiple individual devices, even though it all goes onto a single
network connection].

The other point that immediately comes to mind here is that the Dom0
should definitely have it's own CPU if you're doing a lot of
disk/network IO through it. 

--
Mats 



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