xen-blkfront: weird behavior of "iostat" after VM live-migrate which xen-blkfront module has indirect descriptors

[Ouyang Zhaowei (Charles)]

Hi Roger,

We are testing the indirect feature of xen-blkfront module these days.
And we found that, after VM live-migrate a couple of times, the "%util" of iostat keeps being 100%, and there are several requests stock in "avgqu-sz".
We have checked some later version of Linux, and it happens on Ubuntu 14.04, Ubuntu 14.10 and RHEL 7.0.

The iostat shows like below:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.00    0.00    0.00    0.00    0.00  100.00

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              0.00     0.00    0.00    0.00     0.00     0.00     0.00     4.00    0.00    0.00    0.00   0.00 100.00
dm-0              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
dm-1              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

Could you tell us that why is this happening, is this a bug?

Thanks!

Ouyang Zhaowei

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