Xen users - Apr 2009 - poor I/O performance with PV drivers and I/O emu

Hello,
I wanted to test the disk I/O on my virtual machines using PV drivers
over HVM virtualization to see if I could get better disk IO perormances.
So I compiled and loaded the xen_vnif xend_vbd drivers ecc on my domU
and the network got a real burst in performances. The problem is disk I/O
here is my guest configuration


name = "wn-test-01"
builder = "hvm"
memory = "2048"
disk = [ ''phy:/dev/VolGroup00/test-1_disk,xvda,w'' ]
vif = [ ''type=netfront, mac=00:16:3e:00:00:1f, bridge=eth0'', ]
kernel = "/usr/lib/xen/boot/hvmloader"
device_model = ''/usr/lib64/xen/bin/qemu-dm''
vnc=1
vncunused=1
vnclisten="0.0.0.0"
vncconsole=1
apic=1
acpi=1
pae=1
usbdevice=''tablet''
boot=''nc''
vcpus=1
on_reboot   = ''restart''
on_crash    = ''restart''


I am using a physical partition and exporting the disk as a xvda disc on the
domU.
The domu can see the disk as a xvda.
So I did a test with bonnie++ and I got the same poor results as I have with
IOEMU.
Am I missing something in the configuration or the performances are so poor also
using PV drivers for disk IO ?? is this normal ?

Sequential output around 4MB/s, Sequential input about 50MB/s

I got the same poor results using IOEMU standard emulated disks on domU.

Any hints or suggestions to leverage paravirtualized drivers performances for
Disk I/O
on HVM virtualization ?


thanks


Rick



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On Wed, Apr 15, 2009 at 2:36 PM, Riccardo Veraldi
<Riccardo.Veraldi@cnaf.infn.it> wrote:
> I am using a physical partition and exporting the disk as a xvda disc on
the
> domU.
> The domu can see the disk as a xvda.
> So I did a test with bonnie++ and I got the same poor results as I have
with
> IOEMU.
> Am I missing something in the configuration or the performances are so poor
> also
> using PV drivers for disk IO ?? is this normal ?
Are you SURE domU actually USE xvda?
Last time I check, by default both ide driver (or whatever driver
handles the default IOEMU disk) and xen-vbd will detect the disk,
resulting it to be detected TWICE as hda AND xvda. If you use LVM on
domU, it uses the first device it sees (hda), which would explain the
bad performance.

You might want to try :
- assign new disk as xvdf
- partition and mount that disk on domU (without using LVM)
- run bonnie++ on that disk

If it gives good result, I can tell you later how to make ide driver
ignore the disk. If it still gives bad result, my guess is something
else is wrong with your setup.

Regards,

Fajar

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On Wed, 15 Apr 2009, Riccardo Veraldi wrote:

<snip>
> Sequential output around 4MB/s, Sequential input about 50MB/s
a 12 to 1 ratio in read to write speed is probably NOT related to your 
drivers... drivers should have aproximately the same overhead on a read or 
a write. I would guess that there is something strange in your actual 
storage. I think you need to benchmark /dev/VolGroup00/test-1_disk from 
dom0 and figure out what your I/O system is capable of.

-Tom

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