Best Value Extend Storage with XenServer

[FragglePete]

We've been running XenServer for a couple of years now, have in total 2 XenServers; one running 4 VMs and the other running 5 VMs - both have been rock solid, run flawlessly and for free using the Free Edition.

 

All storage is on the local machine itself; no fancy SAN networks, NAS units, etc.... yet! One VM is the main file server and we're getting uncomfortably close to filling up so its time to look at additional storage.

 

Now, fancy SAN network would be lurvely, but I believe out of our price range (have been told budget is going to be laughable this year). So I've finally gotten round to looking at iSCSI and understanding it now and been playing around with FreeNAS and hooking up a 2008 Server to it on some test machines here.

 

I'm looking at possibly getting something like a Netgear ReadyNAS 3200 (or maybe just a 2100) and using iSCSI to attach to it to give us additional storage for the next lot of students coming this year and moving some other shares over to it. The question would be; would I be better hooking the NAS unit via iSCSI on the Virtual Machine itself? Or just adding it as 'New Storage' on the XenServer Host itself using iSCSI which would then be just assigned to a VM as storage as required?. I'd imagine the latter, but I'm just clearing up the scenerio in my mind.

 

Would I also be right in understanding that if I'm going to access this NAS from multiple servers (unlikely), then this is entering SAN technology which would require it having its own seperate switch and links to the servers instead of just sitting on the main core?

 

Would these ReadyNAS units be up to the job? Read good things so far - wouldn't use it to host VMs though at this stage.

 

Thanks in advance

 

Pete

[teejay]

Why not just use the NAS capability of the ReadyNAS and host the file shares directly on that, it would certainly be quicker than running it as iSCSI through your virtualised servers.

[plexer]

You may be better of looking into using a nas and nfs for your xenservers connected through a dedicated switch.

 

Ben

[FragglePete]

Why not just use the NAS capability of the ReadyNAS and host the file shares directly on that, it would certainly be quicker than running it as iSCSI through your virtualised servers.

 

Would that look after NTFS file permissions doing it that way?

 

Pete

[Guest Guest]

You're getting the terms NAS and SAN abit confused. They are nothing to do with whether the "box" is designed to be a dedicated SAN/NAS, they are how the data is accessed. NAS is file level access by the "boxes" OS presented to the client via SMB (windows share) or NFS etc. SAN is block level access where all the "box" does is present the hard drives as raw storage to the client device, like iSCSI. Using freenas/readynas to present the storage via iSCSI actually makes it a SAN.

 

 

The recommeded practice is to have a seperate switch between the xen hosts and the SAN. In practice you can get away with it being VLANed. The main considerations are latency/throughput on the transfer of data between the hosts and the SAN, ie the NICs and the switch, and the throughput/IOPS of the SAN, ie the harddrives and the RAID controller. The main thing here is to choose the right RAID for your situation, and having as high spindle (hard drive) count as possible. In regards to to the transfer of the data accross the network; you want a dedicated path between the SAN and the hosts. The SAN wants to have 2+ NICs which deal with teh iSCSI and the iSCSI alone, and one for management. The hosts want 1+ NIC(s) dedicated to iSCSI and 2+ NICs for management and the actually connection to your main network.

 

Here a pic of the topology.

http://www.av-digital.com/Images/iSCSI%20Topology2.jpg

 

Above the "gigabit network" is your main LAN. The servers are your Xen hosts, and the box at the bottom represents the SAN. In this diagram there are 2 switches which provide 2 paths for the data, meaning if a switch goes down there is another path for the data to travel. As you can see the main network is not connected to the SAN network at all. All transfer of data goes via the Xen hosts.

 

This brings me onto the next consideration; "putting all your eggs in one basket". Each of the single devices is a single point of failure. If the SAN goes down your whole network goes down. If the only switch goes down your whole network goes down. If the physical cabling goes down... This is why it is best practice to have 2 of everything, but in schools this may be something you can't afford, its up to you to weigh the benifits of a virtual/SAN environment ie costs, power efficiancy, etc, against the potential of losing your whole network should something go wrong.

 

Theres absolutely tonnes of info on this forum conserning SANs so I'll leave it as that.

Edited February 2, 2011 by Guest

[teejay]

Would that look after NTFS file permissions doing it that way?

 

Pete

 

Yes, it supports CIFS/SMB and has active directory integration. You'll get better performance than running as a SAN for file serving as it will have much lower latency on the ip stack. You also haven't got the fun and games of extra switches/vlans to configure as per j17sparkys diagram.

It certainly makes life easier to manage this way, leave your application servers virtualised and let your dedicated storage box look after the file shares.

[FragglePete]

This is all good stuff, its cleared a few things up in my mind - so thanks to all; this is what I needed. Time to play with a few more things and make a decision.

 

Pete