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Re: [Xen-users] File-based domU - Slow storage write since xen 4.8


On 07/17/17 18:46, Roger Pau Monné wrote:
> There are not a huge amount of changes to blkback, so it would be
> helpful if you could pinpoint the change that introduced this
> regression for you, or if you can get a kernel version that introduced
> the regression that would also be helpful.

I used git bisect on the linux-stable source tree to build (a lot of)
tests kernels, and was able to find this commit as the first one
introducing the regression :

d2081cfe624b5decaaf68088ca256ed1b140672c is the first bad commit
commit d2081cfe624b5decaaf68088ca256ed1b140672c
Author: Keith Busch <keith.busch@xxxxxxxxx>
Date:   Tue Jan 12 15:08:39 2016 -0700

    block: split bios to max possible length

In term of kernel version, the first one showing bad performances in my
case is 4.4.2 (with, obviously, 4.4.1 working as expected).

Interestingly, this commit is an improvement of
d3805611130af9b911e908af9f67a3f64f4f0914, which is present in 4.4-rc8
but do not show any performance issue in our case.

I can also confirm that this issue is still present in the latest
unstable kernel (we tested 4.13-rc1).

I think I have reached a point where more competent people should
investigate, but I'll be glad to help if needed.

Best regards,

Benoit Depail
Senior Infrastructures Architect
NBS System

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