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

[Joost Roeleveld <joost@xxxxxxxxxxxx>]

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)

> I already tried the ext4 options barrier=0,data=writeback , it doesn't
> realy change anything.
> I tried too with ext2 and ext3 , it's the same.

To avoid any "issues" with the filesystem, what does "hdparm -Tt <device>" give 
you?

> Is this normal ?

Some drop, yes. loosing 90% performance isn't

> If not what do you think the problem is?

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.

--
Joost Roeleveld

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