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I have several Xenserver boxes running (very well). Most of ours are setup with a RAID1 with two mechanical 7200rpm enterprise sata drives, or RAID10 with 4 7200 enterprise sata drives. The sata drives were selected due to high capacity for the dollar and are WD RE4's, so they are of good quality and decent mechanical drive performance.

I have extra budget to get some nice things for my department, and we were considering getting some SSD's for our virtualization infrastructure. The question is:

Will Xenserver benefit from the SSD's? This isn't such a simple question as it gets complicated when you think out the various possibilities for storage.

We don't have the budget to setup a proper SAN(s) so network storage is out of the question for the time being (guest os's will be stored on the host).

There is no question that the VM's themselves would benefit in performance if they resided on the SSD's, but due to low capacity of SSD's, we were looking at the possibility of having Xenserver itself run off a SSD, however we are questioning if this would have any benefit (does dom0 need high i/o performance? does it matter? or will VM's only really benefit from SSD performance?).

Xenserver configs are fairly basic so having it run from a single disk is not a worry for us (we would just throw together a fresh install then import the VM's from the storage array).

Please advise. (but please don't just say, "yes of course SSD's are better", because it depends how they are used).

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closed as not constructive by mgorven, mdpc, Khaled, Scott Pack, Michael Hampton Mar 16 '13 at 23:45

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Well how many VM's are you running, per server, and how large are these VM's? What's your storage pool size? What's your planned growth? –  David George Mar 14 '13 at 15:40
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Maybe if you said what the virtualized servers do, people could give answers.. if they're DNS servers, SSDs won't help much, if they're Percona DB servers I'd say they're going to obtain much more. –  NickW Mar 14 '13 at 15:40
    
They are various -- we still have several "bare metal" machines that we plan to migrate in the coming future. Our VM's include domain controller, DNS server, mail server (zimbra), file server, application server for intranet users, application server for internet users (low load), webserver, video content web server, temperature monitoring, cacti (snmp graphing), etc. Pretty run-of-the-mill stuff. We already know there is a performance gain to be had by using SSD's for the VM storage, but what if we used SSD's for the hypervisor itself? Does it matter? Does Dom0/DomU need high I/O performance? –  SnakeDoc Mar 14 '13 at 15:52
    
Just to clarify (ran out of room) - we would be separating Xenserver from the storage pool and have it run from its own dedicated drive (possibly an SSD as we are discussing here). This would leave a RAID array (either 1 or 10) for the storage pool, and then the hypervisor would run on its own disk (SSD) separated. This would allow us to easily swap out Xenserver installations and then re-import the vm's in the event of a disaster, etc. –  SnakeDoc Mar 14 '13 at 15:56
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It sounds like a good idea to separate hypervisor and guests, as you say for disaster recovery etc. As for speed increase in my (albeit limited with Xen) experience Xen itself doesn't do much disk access (as you rightly pointed out in your question the VMs are the ones that do). It should start a little quicker but I don't see much in the way of ongoing runtime speed benefit. –  Dave Mar 14 '13 at 16:05

2 Answers 2

up vote 2 down vote accepted

If you really really want to get SSDs you should think about the workloads you are running on XenServer and which ones would benefit from the much higher IOPS and lower latencies - typical candidates are database servers and high traffic mail servers.

As dunxd already said, don't waste money putting the hypervisor on an SSD... I'd rather put it on some 3.5" floppy disks in raid :)

Seriously speaking, where does the SSD thing comes out from? They're just a nice toy you want or you did some analysis and found that your current storage performance is limiting your systems?

Also, SANs will give you many many benefits, you should consider diverting there your budget or keeping it in your pockets until you have enough for a SAN. They are also less expensive than some people might think - keep in mind you could buy some "good enough" SAN (like these) without fully populating it with disks, and add new disks as you need them, choosing from time to time if you need more space (7200rpm nearline disks) or performance (10k, 15k disks) or absolute l33t performance (SSDs), and mix and match them as you need. (we are actually looking at buying an MD3220i with dual controllers, some 15k disks for our VMs, some 7k2 disks for storing backups, logs, and other slowly moving data, and we'll have plenty of unused slots to fill in the future with whatever we'll need).

If you're into that sort of stuff, you can also buy a new server with lots of disks slots and build up a SAN with FreeNAS or similar stuff.

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thanks for the input. We have extra budget that is a use-it-or-loose-it sort of thing (so much waste!, reminds me of our government). So we are going to spend every penny possible. In any regard, the SSD thing came about since we already know our mechanical drives are the bottleneck especially on our 2 disk raid setups (raid 1) -- not enough spindles to keep up with high load I/O times, sometimes even running updates on multiple vm's at once can slow things down. We plan to move to a SAN setup when we can, it will yield more benefits than just performance. –  SnakeDoc Mar 15 '13 at 15:36
    
ran out of room - I've used with a vmware setup before two 2u Rackable Systems servers converted to SAN's in a DRDB HA failover setup and iSCSI'd them to the vmware boxes... worked like a charm. This is similar to what we plan, but thats a ways off. From the sound of it, there will be zero gain from having the hypervisor on a faster disk (we don't care about hypervisor boot time). I think we'll explore getting enough SSD's to move our guest vm's over to them to gain the most performance. And then have our hypervisor run from it's own mechanical disk separated from the storage pool. –  SnakeDoc Mar 15 '13 at 15:39

Getting SSDs just for the hypervisor is a waste of money. It should be running from memory most of the time, so you would just see faster boot time. Unless faster boot time for a host is of critical importance to you. In most cases I would expect you would get better bang for your buck elsewhere.

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