More virtualisation questions :)

[DMcCoy]

We simply use DRBD to mirror volumes. When data is written to one volume, DRBD copies it over the network to another. Having a dedicated network link is nice (and very cheap - just normal £10 gigabit network cards and cable is fine) but not absolutely necessary. DRBD's manual is very good, especially for an open source project. DRBD can mirror standard block devices and in turn presents its mirrored devices as standard block devices, so it can be slotted in in the "storage protocol stack" along with encryption and filesystem-based anti-virus if you like.

 

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David Hicks

 

Although the issue with that is you have doubled your hardware and storage requirements, along with not having hot migration of the machine state itself. It's also not terribly efficient on a large scale.

 

It is a lot cheaper from a VM software point of view though.

[dhicks]

Although the issue with that is you have doubled your hardware and storage requirements

 

You've doubled your requirements for harddrives and decent RAID controllers, but you'd have needed machines to do processing anyway. You also have fully redundant hardware and local access speeds to storage.

 

along with not having hot migration of the machine state itself.

 

Xen supports live VM migration for free.

 

It's also not terribly efficient on a large scale.

 

I agree - at some point in scale (measured by cost of equipment), it's worth centralising/decoupling your storage. I don't know where that point is, but I'm guessing it's somewhere past where most schools would be interested in.

 

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David Hicks

[DMcCoy]

You've doubled your requirements for harddrives and decent RAID controllers, but you'd have needed machines to do processing anyway. You also have fully redundant hardware and local access speeds to storage.

 

 

 

Xen supports live VM migration for free.

 

 

 

I agree - at some point in scale (measured by cost of equipment), it's worth centralising/decoupling your storage. I don't know where that point is, but I'm guessing it's somewhere past where most schools would be interested in.

 

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David Hicks

 

 

Does it work without a shared storage medium?

 

You also keep saying centeralised storage is not something most schools would be interested in, but there are a great many of us here who have already done so. It certainly wasn't common when I did ours some years ago, but it's worth looking at for any large school these days.

[wesleyw]

Xen supports live VM migration for free.

 

 

Xen Express doesn't only the paid for versions?

[webman]

Xen Express doesn't only the paid for versions?

 

I think dhicks means it is possible because he uses DRBD. Essentially he has a second set of servers whose disks are an exact replica of that of the primary set of servers; all running Xen and their respective virtual servers - stored on the 'local' server HDs.

Edited November 19, 2008 by webman

[dhicks]

Does it work without a shared storage medium?

 

Live migration? Sure. You have to have the same data available on both machines somehow, of course - that's what our DRBD mirrored volumes are for. You also have to make sure both sides of the DRBD mirror are set to writeable - the default is to have one writeable, one read-only. Then it's just a case of doing "xm migrate --live ...".

 

You also keep saying centeralised storage is not something most schools would be interested in, but there are a great many of us here who have already done so. It certainly wasn't common when I did ours some years ago, but it's worth looking at for any large school these days.

 

There's got to be a cut-off point somewhere, I just know it - some point where having a central SAN is better value for money than mirrored volumes. Right, time for some calculations...

 

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David Hicks

[dhicks]

Xen Express doesn't only the paid for versions?

 

I'm starting to get the feeling we're talking about slightly different products here. I'm talking about the open source version of Xen, in particular the one available simply by choosing the "virtualisation" option when installing Red Hat / CentOS. I know Ubuntu has support for Xen, too, but the release I tried (7.04?) crashed.

 

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David Hicks

[wesleyw]

Ah yes we are I'm talking about the XenServer Express edition sold by Citrix based on Xen open source product. Sorry wires crossed.

 

Talking of Citrix anyone know of any deals going on as I was quoted £156 per license (looking for 130 licenses roughly £20,000) not exactly cheap.

 

Wes

[DMcCoy]

Live migration? Sure. You have to have the same data available on both machines somehow, of course - that's what our DRBD mirrored volumes are for. You also have to make sure both sides of the DRBD mirror are set to writeable - the default is to have one writeable, one read-only. Then it's just a case of doing "xm migrate --live ...".

 

 

 

There's got to be a cut-off point somewhere, I just know it - some point where having a central SAN is better value for money than mirrored volumes. Right, time for some calculations...

 

--

David Hicks

 

 

I didn't realise it could quick migrate when disk mirroring, quite handy that (at least in the open source version). I believe it's hyper-v thats missing a quick migration feature completely.

 

One of the reasons I've been using vmware for so long is this:

 

None of my blade CPUs support virtualisation extensions, this still works fine for vmware but rules xen and hyper-v out, also means I can't have 64 bit VMs. The cpu's themselves are actually 64 bit, so I have 8GB ram still, they are very early 64 bit single core xeons.