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We're a little behind in the virtualization game, I'll have to admit. We currently have nothing virtualized, but I'm trying to change that.

We want to get our feet wet with virtualization and get a feel for it, so I am going to propose for next year we get a server and virtualize 3 rather under-utilised (and incredibly old) non-critical servers.

I know virtualization is all about SANs, but unfortunately we don't have one (all our servers are DAS). The question I have is if I get a server (probably a HP DL380 of some description) with DAS, are the guest VMs (under-utilised don't forget) going to suffer badly as a result? My current thinking is to put Hyper-V Server 2008 R2 as the host, if it makes any difference.

I'll give a quick run down of what the 3 servers do below. I know this isn't an exact science, but roughly speaking - do you think DAS will be OK?

Server 1
Runs a legacy HR system which is accessed maybe once or twice a week. This has a SQL Server 2000 backend which also runs on this server. This thing is archaic (I don't know when HP stopped branding ProLiants Compaq, but this proudly displays the Compaq logo on the front).

Server 2
Runs some emulated printers for our AS/400 and prints the output to some real printers. This runs happily on an oldish PC with W2K3 and 2GB RAM.

Server 3
Runs a server for a BI product, however all it actually does is at 4:00AM imports a ton of data from our AS/400 and compiles it into a file (this takes about half an hour). About 15 users then periodically (maybe once every few days) download this compiled file (~100MB) to their local computer. The download is user initiated, so there's no real defined access pattern.
This is also on a PC running 2K3 but with 3GB RAM.

We've looked at virtualization several times now, but because of our lack of SAN and the many advantages it brings, we have dismissed virtualization as an option each time.

I know some of the pitfalls of putting these VMs on DAS (lack of HA etc) - and that is OK for these machines. If the host were to fail, it would be an inconvenience rather than a full blown disaster.

I'm looking at this more of a consolidation project rather than anything else. The 3 servers are slated for replacement next year anyway, and like I said they are getting rather old, sit mostly idle and are big tower machines using a lot of space and electricity. In my opinion, 3 new servers (however basic) is still overkill for machines that sit doing very little all day.

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I don't have an answer for you but I have an opinion on virtualization: Virtualization is not a neccessity. There are many reasons an organization may choose to virtualize their IT infrastructure but doing it because everyone else is doing it or virtualizing just for the sake of virtualizing aren't valid reasons in my book. Virtualization should be viewed like any other business endeavor: Is there a valid business or technical reason for it, does it give us some competitive advantage, does it reduce our TCO, does it increase our ROI, etc., etc. No offense intended. –  joeqwerty Sep 2 '10 at 3:12
In addition, your question implies that you haven't thought this through from a business or technical perspective and haven't validated your reasons for virtualization. You're considering virtualization on non-optimal hardware and you're guessing at which servers would be good candidates for virtualization, which tells me this is a "throw it against the wall and see if it sticks" attempt at virtualization. My advice would be to read some of the many white papers regarding virtualization and conduct an analysis on your environment to see if virtualization can actually benefit your organization. –  joeqwerty Sep 2 '10 at 3:21
@Ben: As in my first comment, no offense is intended by any of my comments (including this one). It's my opinion that as a sysadmin I need to pursue projects that have real world benefits for the business and are targeted to address specific, identified needs. I don't implement things just because I can, or because I think they're cool, or because everyone else is doing it. –  joeqwerty Sep 2 '10 at 3:24
Storage performance is unlikely to be any kind of an issue for this sort of "low-hanging fruit" virtualization project. You will need to look at your actual performance requirements (refer to Jake's answer below), but, it is likely that something ludicrously cheap/simple (e.g. four 2TB 7.2k SATA drives in RAID10) would be more than adequate for these machines' VHDs. It is unclear whether you meant internal or external DAS; if you meant external, be sure to consider the possibility that you may be able to satisfy your storage capacity/performance requirements within a single 2U server case. –  Skyhawk Sep 2 '10 at 6:44
@joeqwerty - no offence taken, I appreciate everybody's opinion. I have added an update to hopefully better clarify what I'm trying to achieve. I assure you it's not just to "keep up with the Jones'", I believe that I present a valid business case that will save us some money (on the cost of 3 servers, maintenance contracts and electricity) going forward. –  Ben Pilbrow Sep 2 '10 at 7:18
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4 Answers

up vote 6 down vote accepted

Your question can be answered partly by looking at the performance counters for I/O in the machines that you're trying to migrate to virtual. Look at how many I/O operations per second (IOPS) each requires. Remember that each physical disk is good for a few tens of IOPS. Now size your virtualization system to support that level of I/O. You'll know how many "spindles" you need. Use a RAID controller to spread the load across the disks.

Note that none of this said anything about a SAN. SANs are great, but they mostly provide things that you didn't mention: high availability, backup, storage tiering, fault tolerance, cross-site disaster recovery, rapid deployment, etc.

The most notable part of all of that is that if you use DAS, the failure of the physical server that the DAS is attached to constitutes a failure of all the VMs that were hosted on it. With a SAN, using a cluster of compute hosts all attached to the SAN, you can survive the failure of the physical host by moving the VM to a new host.

Another strategy for surviving failure of that physical host is to back up the apps and OSes within the VMs frequently, or at least frequently enough that you won't mind losing things that happened after the last backup.

Your choice. DAS can give you the performance you need (given what you have described.)

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+1 - In some cases questions about performance may be orthagonal to the DASD vs SAN question. Lower-end SAN solutions (iSCSI on SATA disks) may not be able to out-perform 15K RPM SAS disks on a hardware RAID controller directly-attached. –  Evan Anderson Sep 2 '10 at 1:20
nearly +1. Discs are good for a few HUNDRED IOPS. Depending on the disc. –  TomTom Sep 2 '10 at 7:51
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1. Performance In my experience, SAN storage and DAS storage have different characteristics which make performance hard to measure objectively, since workloads tend not to rely on just one metric (e.g. access time), but rather a variable blend of access time, throughput, read vs write, etc. It's a very complex picture. The main benefits of a SAN, IMO, are having your data available to several hosts simultaneously (allowing for clustering/high availability), and unifying the storage of your data allowing for easier management and maintenance. So long story short - I wouldn't sweat performance all that much for DAS.

2. SAN Availability & Cost

You don't have to go with off-the-shelf discrete SAN solutions. Many folks are building their own since a collection of regular server parts can get you a fully functional SAN system. Here's one route to that:

  1. Pick the server that you own that can take the most disks with the highest total capacity
  2. Strip out all but a couple of Gb of RAM
  3. Install OpenFiler
  4. Configure some volumes and expose some iSCSI LUNs
  5. Hook the server into an isolated gigabit VLAN
  6. Plumb your virtual hosts into the same storage network
  7. Point the hosts at the target LUNs
  8. Start adding VMs to your SAN

Vendor support can be purchased for OpenFiler, and your hardware, if your company insists on these things. You can also configure multiple OpenFiler hosts with DRDB to give you high availability and redundancy options.

We've run one of these in production for over a year now, and it's been very solid.

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DAS normally, should be faster than some FC SANs, and definitely faster than iSCSI SANs. You're getting a SAS link speed (3Gbps and in the newer boxes - 6Gbps)

The downside to DAS is not speed or stability, it's scalability, because the amount of ports on a DAS is limited, so you can't connect too many hosts to it.

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When we started with virtualization in 2007 we had drbd in mind. So we started with a server-pair that had 5 internal 3,5" SAS-disks (configured as RAID 5 providing 500 GB netto storage). Since our database-servers run in a different cluster (SAN-attached) our virtual systems don't do much disk io - which makes this a possible scenario - even with 100 MBit Ethernet for drbd replication.

With that setup we were able to provide over 20 HA-VMs (XEN paravirtualized). Disk IO or disk performance was never a problem. DAS is even faster nowadays than local storage. So for your setup with just three low performance, low disk-io servers I would say that even DAS is oversized.

Since you use mainly W2K you could play with Linux KVM or Windows Hyper-V. Or Windows-Admins love the latter (and use DAS attached to two servers themselves). With Linux KVM I would use DRBD as ha-storage backend (DAS or internal disks).

Just a note on SAN/DAS: We attached some of our Virtualization Servers to SAN - for those guests that now run databases with heavy IO. But: SAN is not faster than the internal disks - it just distributes io to more disks - the same can be true for DAS.

I see the real advantage on SAN in a different area: It uses its own switching fabric, which has (at my work) nothing to do with the IP routers and switches - and is much more stable. From a cluster/ha point of view you get another - really independent - connection.

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