Re: [Xen-users] Xen4.0.1 : slow Disk IO on DomU

[Erwan RENIER <erwan.renier@xxxxxxxxxxx>]

Le 18/03/2011 09:00, Joost Roeleveld a écrit :
> On Thursday 17 March 2011 18:31:10 Erwan RENIER wrote:
> > Le 17/03/2011 09:31, Joost Roeleveld a écrit :
> > > On Wednesday 16 March 2011 23:31:31 Erwan RENIER wrote:
> > > > Hi,
> > > > When i test the IO bandwidth it's pretty much slower on DomU :
> > > >
> > > > Dom0 read  : 180MB/s write  : 60MB/s
> > > > DomU read  : 40MB/s write :  6MB/s
> > > Just did the same tests on my installation (not yet on Xen4):
> > > Dom0:
> > > # hdparm -Tt /dev/md5
> > >
> > > /dev/md5:
> > >    Timing cached reads:   6790 MB in  1.99 seconds = 3403.52 MB/sec
> > >    Timing buffered disk reads:  1294 MB in  3.00 seconds = 430.94
> > >    MB/sec
> > >
> > > (md5 = 6-disk RAID-5 software raid)
> > >
> > > # hdparm -Tt /dev/vg/domU_sdb1
> > >
> > > /dev/vgvg/domU_sdb1:
> > >    Timing cached reads:   6170 MB in  2.00 seconds = 3091.21 MB/sec
> > >    Timing buffered disk reads:  1222 MB in  3.00 seconds = 407.24
> > >    MB/sec
> > >
> > > DomU:
> > > # hdparm -Tt /dev/sdb1
> > >
> > > /dev/sdb1:
> > >    Timing cached reads:   7504 MB in  1.99 seconds = 3761.93 MB/sec
> > >    Timing buffered disk reads:  792 MB in  3.00 seconds = 263.98 MB/sec
> > >
> > > Like you, I do see some drop in performance, but not as severe as you
> > > are
> > > experiencing.
> > >
> > > > DomU disks are Dom0 logical volumes, i use paravirtualized guests, the
> > > > fs type is ext4.
> > > How do you pass the disks to the domU?
> > > I pass them as such:
> > > disk = ['phy:vg/domU_sda1,sda1,w',
> > > (rest of the partitions removed for clarity)
> > My DomU conf is like this :
> > kernel =  "vmlinuz-2.6.32-5-xen-amd64"
> > ramdisk = "initrd.img-2.6.32-5-xen-amd64"
> > root = "/dev/mapper/pvops-root"
> > memory = "512"
> > disk = [ 'phy:vg0/p2p,xvda,w' , 'phy:vg0/mmd,xvdb1,w', 'phy:sde3,xvdb1,w' ]
> > vif = [ 'bridge=eth0' ]
> > vfb = [ 'type=vnc,vnclisten=0.0.0.0' ]
> > keymap = 'fr'
> > serial = 'pty'
> > vcpus = 2
> > on_reboot = 'restart'
> > on_crash = 'restart'
> seems ok to me.
> Did you pin the dom0 to a dedicated cpu-core?
Nop
> > > Either you are hitting a bug or it's a configuration issue.
> > > What is the configuration for your domU? And specifically the way you
> > > pass the LVs to the domU.
> > As you can see :
> > xvda is a lv exported as a whole disk  with lvm  on it, so xvda2 is a lv
> > from a vg  in a lv ( ext4 =>  lv =>  vg =>  pv =>  virtual disk =>  lv =>vg
> > =>pv =>  raid5 =>disk )
> > xvdb1 is a lv exported as a partition ( ext4 =>  virtual part =>  lv =>  vg
> > =>  pv =>  raid5 =>  disk )
> > xvdb2 is a physical partition  exported as a partition ( ext3 =>  virtual
> > part =>  disk )
> >
> > Curiously it seems the more complicated, the better it is :/
> Yes, it does seem that way. Am wondering if adding more layers increases the
> amount of in-memory-caching which then leads to a higher "perceived"
> performance.
>
> One other thing, I don't use "xvd*" for the device-names, but am still using
> "sd*". Wonder if that changes the way things behave internally?
I doesn't change with sd*
I noticed that the cpu io wait occurs in domU ,nothing happen in dom0

Does someone knows a way to debug this ? at kernel level or in the 
hypervisor ?
By the way how to get the hypervisor activity  i don't think it appears 
in dom0.
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