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

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

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
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