Re: IO speed limited by size of IO request (for RBD driver)

[Steven Haigh <netwiz@crc.id.au>]

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.

> Also, I think we should bring this conversation back to xen-devel.

Agreed.

-- 
Steven Haigh

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