RE: [Xen-users] Poor disk io performance in domUs

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Andrej Radonic
> Sent: 22 June 2007 08:57
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Poor disk io performance in domUs
> 
> Tom,
> 
> > Are the disk I/O numbers quoted below aggregate for all 
> machines, or  
> > just for the mailserver.
> 
> so far we measured separately in dom0 and in the mail-domU.
> The 20 MB/s happen as soon as there is concurrent io from two 
> or more domUs.

That would be a consequence of the two domains causing more
head-movement on the drive(s) than a single domain, so you get more
overhead. So if you get 50MB/s in a single domain, you don't get 25MB/s
in two parallel domains - you get a bit less. That's just what I'd
expect in this situation. [This is because the "disk" for each virtual
machine is in a different part of the disk, so each time the first
domain acesses the disk, it needs a (big) move of the read/write head,
and then another big move when the second domain accesses it's part of
the disk]. 

--
Mats
> 
> Andrej
> 
> > -- -- Tom Mornini, CTO -- Engine Yard, Ruby on Rails 
> Hosting -- Support, Scalability, Reliability -- (866) 
> 518-YARD (9273) On Jun 20, 2007, at 10:49 PM, Andrej Radonic wrote:
> >> > Hi,
> >> >
> >> > in a previously well behaving xen setup we are suddenly  
> >> > experiencing massive loss of disk performance.
> >> >
> >> > Setup: two identical machines (dell, 2 quadcore cpus 
> each, 8 GB RAM  
> >> > each), attached to *one* SAS storagebox (Raid5 SAS 
> disks), CentOS  
> >> > 5, Xen 3.04 running three domUs each.
> >> > Storage partitions are exposed as block-devices to the guests.  
> >> > Filesystem is ext3.
> >> >
> >> > Since a domU running a rather heavily loaded mailserver 
> (many small  
> >> > files, many reads/writes) has been added, io performance has  
> >> > deteriorated significantly.
> >> >
> >> > In dom0 about 100 MB/s can be achieved, in domU only 
> about 50 MB/s.  
> >> > This looks even worse when there are massive concurrent 
> / parallel  
> >> > io jobs, then throughput goes down to 20 MB/s.
> >> >
> >> > During all this the cpus are mostly idle...
> >> >
> >> > Any ideas? I know some performance degradation is to be 
> expected  
> >> > but shurely not on this scale?
> >> > Thanks,
> >> > Andrej
> >> >
> >> >
> >> > _______________________________________________
> >> > Xen-users mailing list
> >> > Xen-users@xxxxxxxxxxxxxxxxxxx
> >> > http://lists.xensource.com/xen-users
> 
> 
> 
> interSales AG Internet Commerce
> Andrej Radonic
> Vorstand
> 
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> http://lists.xensource.com/xen-users
> 
> 
> 



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