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Xen 4.7.2: kernel-4.9.34-gentoo poor disk performance dom0
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ElChupacabra
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Joined: 10 Sep 2010
Posts: 10
Location: Belgium

PostPosted: Fri Aug 04, 2017 8:56 pm    Post subject: Xen 4.7.2: kernel-4.9.34-gentoo poor disk performance dom0 Reply with quote

Hi, guys i m banging my head on the wall on this one ;).
I m having really poor disk IO with my current kernel configuration, not matter if it run under Xen or not and i got no clue why.
When i boot on the gentoo live (kernel 4.9.34-gentoo too), i got expected disk performance.
The strange thing is that hdparm report normal disk performance but not dd neither the zfs pools.

Code:
# hdparm -t --direct /dev/disk/by-vdev/sys-1
/dev/disk/by-vdev/sys-1:
 Timing O_DIRECT disk reads: 1164 MB in  3.00 seconds = 387.64 MB/sec

Code:
# dd if=/dev/disk/by-vdev/sys-1 | pv | dd of=/dev/null
^C18MiB - 0:00:08 @ [87.4MiB/s][89.9MiB/s]


some logs:
dmesg: https://pastebin.ca/3850880
xl dmesg: https://pastebin.ca/3850876

the kernel config:
https://pastebin.ca/3850881

i always strip down the kernel settings quite alot, but this time i want too far i guess, or i got something offending.

Thx in advance for the help.
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ElChupacabra
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Joined: 10 Sep 2010
Posts: 10
Location: Belgium

PostPosted: Sun Aug 06, 2017 7:05 pm    Post subject: Reply with quote

After more investigation it seems the pipes and the use of pv is slowing down the transfer.
i am able to reach the expected transfer rate with my kernel config without Xen.
But under the xen hypervisor, the transfert rate is still cut in half.
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