Re: [Xen-devel] IO speed limited by size of IO request (for RBD driver)

[Steven Haigh <netwiz@xxxxxxxxx>]

On 27/04/2013 5:06 PM, Roger Pau Monné wrote:
> On 27/04/13 03:57, Steven Haigh wrote:
> > On 27/04/2013 12:16 AM, Steven Haigh wrote:
> > > On 27/04/2013 12:06 AM, Roger Pau Monné wrote:
> > > > On 23/04/13 21:05, Steven Haigh wrote:
> > > > > Sorry - resending this to Felipe as well - as I started talking to him
> > > > > directly previously.
> > > > >
> > > > > Felipe, to bring you up to date, I've copied over the blkback files from
> > > > > Rogers indirect kernel over the vanilla 3.8.8 kernel files, built and
> > > > > tested. Results below:
> > > Bringing this into context in a nutshell - results showed about 5MB/sec
> > > improvement when using buffered disk access - totalling ~57MB/sec write
> > > speed vs ~98MB/sec when using the oflag=direct flag to dd.
> > >
> > > When talking about back porting a few indirect patches to mainline
> > > blkback (3.8.8 atm):
> > > > > On 24/04/2013 4:13 AM, Roger Pau Monné wrote:
> > > > > > I think it requires a non-trivial amount of work, what you could do
> > > > > > as a
> > > > > > test is directly replace the affected files with the ones in my
> > > > > > tree, it
> > > > > > is not optimal, but I don't think it's going to cause problems, and you
> > > > > > could at least see if indirect descriptors solve your problem.
> > > > > Ok, I copied across those files, built, packaged and installed them on
> > > > > my Dom0. Good news is that its a little quicker, bad news is not by
> > > > > much.
> > > > Could you try increasing xen_blkif_max_segments variable in
> > > > xen-blkfront.c to 64 or 128? It is set to 32 by default. You will only
> > > > need to recompile the DomU kernel after this, the Dom0 is able to
> > > > support up to 256 indirect segments.
> > > I'll have to look at this. All DomU's are Scientific Linux 6.4 systems -
> > > so essentially RHEL6.4 and so on. I haven't built a RH kernel as yet -
> > > so I'll have to look at what is involved. It might be as simple as
> > > rebuilding a normal SRPM.
> > Ok, I've had a look at the RH xen-blkfront.c - and I can't see any
> > definition of xen_blkif_max_segments - or anything close. I've attached
> > the version used in the EL6 kernel from the kernel-2.6.32-358.6.1.el6 srpm.
> >
> > Any ideas on where to go from here?
> I thought you were using the 3.8.x kernel inside the DomU also, if you
> are not using it, then it's normal that there's no speed difference, you
> have a Dom0 kernel that supports indirect descriptors, but your DomU
> doesn't. You must use a kernel that supports indirect descriptors in
> both Dom0 and DomU in order to make use of this feature.
Ahhh - sorry - I should have been clearer - The Dom0 is kernel 3.8.x 
(3.8.8 right now) - however the DomUs are all stock EL6 kernels.
Hmmmm - I believe the kernel I build for Dom0 *should* work as a DomU. 
I'll do some more experimentation and see if I can get it working 
properly as a DomU kernel.
--
Steven Haigh

Email: netwiz@xxxxxxxxx
Web: https://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
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