RE: [Xen-users] Disk IO trouble in Xen

["Ross S. W. Walker" <rwalker@xxxxxxxxxxxxx>]

Greg Hellings wrote:
> 
> I started this thinking I was having an IO problem.  Now, I'm 
> not so sure.
> I am using a pre-allocated disk, not a sparse file.
> 
> Write performance seems about the same for both
> 
> Dom0
> [root@localhost ~]# dd if=/dev/zero of=/tmp/dummy count=2097152
> 2097152+0 records in
> 2097152+0 records out
> 1073741824 bytes (1.1 GB) copied, 29.8368 seconds, 36.0 MB/s
> 
> DomU
> [root@localhost ~]# dd if=/dev/zero of=/tmp/dummy count=2097152
> 2097152+0 records in
> 2097152+0 records out
> 1073741824 bytes (1.1 GB) copied, 30.4716 seconds, 35.2 MB/s
> 
> Iozone shows very similar numbers on both the Dom0 and DomU. The DomU seems
> to freeze periodically and drop network connections.  I assumed this was IO
> related because everything else seems fine and those hdparm numbers are so
> far off.  Also heavy IO related tasks seem slow.  I'm fairly baffled now.
> 

Run a 'vmstat 1' and take a look at the interrupts and io waits.

Maybe the network is generating a high load?

Performance, and performance under load are two different things.


> On 5/2/08 4:04 AM, "Sadique Puthen" <sputhenp@xxxxxxxxxx> wrote:
> 
> > Are you using a pre-allocated image or a sparse file? If 
> the latter it's
> > likely to have performance problems like you found?  What 
> if you test
> > with dd or iozone or bonnie++?
> > 
> > --Sadique
> > 
> > Greg Hellings wrote:
> >> I'm having terrible IO trouble with an image based domu
> >> 
> >> 
> >> This is the performance on the Dom0
> >> [root@localhost /]# hdparm -tT /dev/sda
> >> 
> >> /dev/sda:
> >>  Timing cached reads:   3228 MB in  2.00 seconds = 1617.22 MB/sec
> >>  Timing buffered disk reads:  174 MB in  3.02 seconds =  
> 57.60 MB/sec
> >> 
> >> 
> >> And this is the performance on the DomU
> >> [root@localhost ~]# hdparm -tT /dev/xvda
> >> 
> >> /dev/xvda:
> >>  Timing cached reads:   3336 MB in  2.00 seconds = 1669.98 MB/sec
> >>  Timing buffered disk reads:   26 MB in  3.13 seconds =   
> 8.30 MB/sec
> >> 
> >> 
> >> Is it normal for the performance difference to be so 
> great?  I'm running
> >> CentOS 5 and the DomU image is stored on a LVM ReiserFS 
> formatted partition.
> >> 
> >> Here is the config for the DomU
> >> name = "dddevdb"
> >> uuid = "927f1e915813c2dec60c8f76c2716783"
> >> maxmem = 3072
> >> memory = 2042
> >> vcpus = 2
> >> bootloader = "/usr/bin/pygrub"
> >> on_poweroff = "destroy"
> >> on_reboot = "restart"
> >> on_crash = "restart"
> >> vfb = [ "type=vnc,vncunused=1,keymap=en-us" ]
> >> disk = [ "tap:aio:/var/lib/xen/images/dddevdb.img,xvda,w"]
> >> vif = [ "mac=00:16:3e:62:7a:d1,bridge=xenbr0" ]
> >> 
> >> 
> >> _______________________________________________
> >> Xen-users mailing list
> >> Xen-users@xxxxxxxxxxxxxxxxxxx
> >> http://lists.xensource.com/xen-users
> >> 
> >>   
> > 
> 
> 
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> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users
> 

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