[Bug 212681] I/O is slow for FreeBSD DOMu on XenServer

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=212681

--- Comment #37 from Roger Pau Monné <royger at freebsd.org> ---
(In reply to Roger Pau Monné from comment #33)
So I've run the dc3dd test on a FreeBSD VM, with 4 vCPUs and 4GB of RAM,
against a block device on a spinning disk, nothing fancy, and this is what I
get:

(FreeBSD guest) # dc3dd wipe=/dev/ada1

dc3dd 7.2.641 started at 2017-02-10 16:19:54 +0000
compiled options:
command line: dc3dd wipe=/dev/ada1
device size: 20971520 sectors (probed),   10,737,418,240 bytes
sector size: 512 bytes (probed)
 10737418240 bytes ( 10 G ) copied ( 100% ),  125 s, 82 M/s

input results for pattern `00':
   20971520 sectors in

output results for device `/dev/ada1':
   20971520 sectors out

dc3dd completed at 2017-02-10 16:21:58 +0000

Then I shut down the guest, and ran the same test from Dom0 (Debian Linux 3.16)
against the same exact block device, and this is what I get:

(Linux Dom0) # dc3dd wipe=/dev/dt51-vg/test

dc3dd 7.2.641 started at 2017-02-10 16:31:34 +0000
compiled options:
command line: dc3dd wipe=/dev/dt51-vg/test
device size: 20971520 sectors (probed),   10,737,418,240 bytes
sector size: 512 bytes (probed)
 10737418240 bytes ( 10 G ) copied ( 100% ),  114 s, 90 M/s

input results for pattern `00':
   20971520 sectors in

output results for device `/dev/dt51-vg/test':
   20971520 sectors out

dc3dd completed at 2017-02-10 16:33:28 +0000

So there's a < 10M/s difference, which I think it's fine (at the end there's
always some overhead).

I cannot really explain the results that you get, but I cannot also reproduce
your entire setup here. This is using the 12.0-CURRENT snapshot from 20170203.
I'm afraid that unless we find a way for me to reproduce this, there's no way
that I can try to fix it.

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