This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-users] How many guests

To: Michael Schmidt <michael.schmidt@xxxxxxxxxx>, Xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] How many guests
From: Jonathan Tripathy <jonnyt@xxxxxxxxxxx>
Date: Mon, 07 Jun 2010 08:16:53 +0100
Delivery-date: Mon, 07 Jun 2010 00:18:06 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4C0C9BE8.7050106@xxxxxxxxxx>
List-help: <mailto:xen-users-request@lists.xensource.com?subject=help>
List-id: Xen user discussion <xen-users.lists.xensource.com>
List-post: <mailto:xen-users@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
References: <4C0AD5A6.9040307@xxxxxxxxxxx> <4C0BF478.6040903@xxxxxxxxxx> <4C0BF94E.10806@xxxxxxxxxxx> <201006062321.30750.bart.coninckx@xxxxxxxxxx> <4C0C9BE8.7050106@xxxxxxxxxx>
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100423 Thunderbird/3.0.4
Hi Michael,

You state that iSCSI is reliable but expensive. But isn't iSCSI nearly free?

I agree with you that Fibre Channel systems are very expensive

Would iSCSI over IP be ok?


On 07/06/10 08:12, Michael Schmidt wrote:
This is not completely correct.
With a raid 1, you have the read performance of 2 disks and just the write performance of a single disk.

To the other thinks following this thread:
If you use a network storage, you have a bandwidth limit with the connection. But in the most cases, the raw bandwidth is not the bottleneck (instead of the IOs per second).

Network Storages using NFS or NBD are not stable enough in my eyes.
iSCSI and FC SANs but really stable and expansive as well. But there is another much less expensive way:

You get the most servers with an external SAS port. There you can connect over a SAS link a JBOD with 12 - 16 disk bays (DAS).
This disks can be managed by the servers raid controller.

Best Regards

Michael Schmidt

Am 06.06.10 23:21, schrieb Bart Coninckx:
RAID1 does not perform better than a single disk. It will still depend on what those 5 to 10 VMs would do. It still might be stretching it. For 10 webservers
visited by 5 users per hour: I would say no problem. For 5 heavily used
database servers it will be another story.

I guess the only real way to find out is to put your guests on there and try.
If you clone them, you will know quite fast.

On Sunday 06 June 2010 21:38:54 Jonathan Tripathy wrote:
Thanks Micael,

I understand what you are saying.

With a small setup such as a RAID1 array, how many VMs could I rent out?

It doesn't matter if it's a small number, it's just to utilise the
server a bit.

Think it would cope with 5-10?



On 06/06/10 20:18, Michael Schmidt wrote:
Hi Jonathan,

if you plan to migrate existing physical machines to xen VMs, or you
have some different machines for a comparison,
you can easy get runtime statistics and calculate the usage. Look at
the running iostats and cpu usage.

If you plan to rent generic VMs on this server to customers, you disk
/ raid setup will be absolutely the bottleneck.
A solution at this point is not easy. If you have much write IOs, use
raid 10 with 4 to 8 disks. With many reads - raid 6 or 50 with the
same amount of disks.
In each case i can suggest you 15k rpm SAS disks.

Then you can run 29 VMs. Or 60 VMs with 16GB memory and 2 CPUs.

But note: You cannot set disk priority to the VMs. So if one VM does
heavy disk IO, all off the other VMs slowed down.

Best Regards

Michael Schmidt

Am 06.06.10 20:45, schrieb Jonathan Tripathy:
Hi Michael,

Thanks for your email.

This is just an idea that I have floating around in my head that
maybe I'd like to rent out some VPSs to customers, just to utilise my
machine which will be sitting in a co-lo nearly idle.

I'd give out VPSs with 256MB RAM and probably 5Mbps connection speed.

So the answer is, I don't know what will be running on them, however
I could write up an "acceptable use policy", as well as use some


On 06/06/10 19:39, Michael Schmidt wrote:
Hi Jonathan,

the question is, what a kind of VM?
You can over-utilize a much greater machine with one VM.
Or on the other side, you can run 40 VMs on a shorter machine.

Each ressource can be a bottleneck

- Memory - this is realy easy to calculate: Avaiable minus 768MB
(Reserved for Dom0 should be enugh in this case).
- CPU - Here we need a VM statistic
- Disk Bandwidth - Here we need a VM statistic, but in the most
cases not the bottleneck
- Disk IOPS - Here we need a VM statistic, in the most cases the

What a kind of VMs you plane to run?
Webservers / mailservers / database-servers ...?

Best Regards

Michael Schmidt

Am 06.06.10 00:54, schrieb Jonathan Tripathy:
Hi Everyone,

I have a Dell R210 server which has a Xeon X3430 Quad Core CPU
(2.4Ghz x 4) with 8GB of RAM. I intend to use the H200 controller
in a RAID1 setup

How many VMs do you think I'd be able to run on this machine? Is 20
pushing it?

I'd say most (if not all) guests would be in PV mode.


Xen-users mailing list
Xen-users mailing list
Xen-users mailing list

Xen-users mailing list

Xen-users mailing list

<Prev in Thread] Current Thread [Next in Thread>