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Running anything inside a virtual machine will have some level of performance hit, but how much does it really impact the performance of a database system?

I found this academic reference paper with some interesting benchmarks, but it was a limited test using Xen and PostgreSQL only. The conclusion was that using a VM does "not come at a high cost in performance" (although you may think the actual data says otherwise).

What are the technical, administrative, and other drawbacks associated with running a database within a virtual machine?

Please post answers that can be backed up by objective facts, I'm not interested in speculation or any other semi-religious argument (geek passion is good in many ways, but that won't help us here).

That being said,

  • What issues show up when running database in a Virtual Machine? (please post references)
  • Are those issues significant?
    • Are they only significant under certain scenarios?
  • What are the workarounds?
makerofthings7
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Russ
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7 Answers7

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Though many DB vendors were very slow to do this, nearly all of them now officially support their software running in a virtualized environment.

We run many Oracle 11g instances in linux on top of ESXi, and it is certainly possible to get very good performance. As with all hardware scaling, you just need to make sure that the virtualization host has plenty of resources (RAM, CPU), and that your disk layer is up to the task of delivering whatever IO performance you require.

EEAA
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As ErikA says, this is becoming more and more common. I'm in the SQL Server camp and don't personally have any production systems running in VM's, but I would not be hesitant to (after a little more study on the topic). There are definitely some things to take into consideration before you go down that path, though (at least for SQL Server). Disk IO (as others have mentioned) and memory allocation are just 2 examples. Things will be different between different hypervisors as well.

Brent Ozar is a recognized expert in virtualizing SQL Server, specifically in VMWare. I would highly recommend reading through his material.

http://www.brentozar.com/community/virtualization-best-practices/

squillman
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There is can and then there is should. A corvette can go 150 mph, but should you on public highways? You can harm yourself unnecessarily.

Databases are guest operating systems. By design when they start they grab blocks of a resource and manage it directly for performance reasons. As soon as you make the core operating system of the database server a guest in virtualized hosting environment then you are placing an arbitration layer with the hypervisor between the block allocated element of disk and RAM and the database server. It will slow down. The more inefficient your queries, the more it will slow. These inefficiencies may be masked today on dedicated hardware, but as soon as you introduce arbitration to your dependent resource you are going to find out real fast.

What a lot of bean counters who are demanding virtualization fail to recognize is that database servers, as guest operating systems, offer their own consolidation layer. There is no reason why you cannot move consolidate multiple logical database instances on one physical server, even to the point of moving IP addresses, setting up additional host names, etc..., to allow for this natural coalescing of services to take place. And, with this model not only do you retain the cost savings that the management is pushing for reduced number of physical hosts, but you retain the block access to physical resources without the impingement of the arbitrary hypervisor, which can make beneficial decisions sometimes and not others.

The same holds true for other guest operating systems, like Java. Virtualization solutions are typically busy environments and the hypervisor has to make lots of decisions on who "gets the token" on a resource. Anytime you can eliminate that layer you are going to be better off.

Coalesce multiple instances using the natural guest operating system layer first. Odds are you will be able to hit your platform consolidation and performance targets easier.

James Pulley
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There are two things to realize here:

  • Unit of DB performance per unit of Hardware is a bit lower for a virtualized db. This means you need to buy a little more hardware to get the same level of performance.
  • That doesn't mean the same level or a desired level of performance is unobtainable. The gains you get from improved management and other benefits (like easier HA) often way more than offset the marginally increased hardware costs.

That said, where I work our Sql Server installation is one of only two servers that I have no intention of virtualizing any time soon (the other is the primary DC).

Joel Coel
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Running SQL Server is a VM will be fine, provided that you can provide enough resources to the VM to run your application. If in the physical world you need 24 cores and 256 Gigs of RAM then you need to provide 24 vCPUs and 256 Gigs of RAM in the virtual world.

I just wrote an article in last months SQL Server magazine all about running SQL Server under VMware's vSphere.

mrdenny
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I run two databases, one PostgreSQL and the other MySQL, in a virtual environment (Xen) where the dom0s are highly available. domU file systems are all located on an iSCSI SAN LUN, carved up with LVM2 logical volumes. The MySQL database is solely for Cacti, and so does not see very much usage at all, and is located on the iSCSI LUN as well.

The PostgreSQL database is the database for our staging environment, and therefore sees higher utilization than the MySQL db. For this reason, the database is located on a local RAID10 set, and DRBD replicated to the second cluster node. However, in terms of real load, this staging database doesn't see very high load at all. Which, in my opinion, makes it a good/great candidate to virtualize.

Some of the benefits to our organization was the reduced power consumption, saved rack space, and less hardware administrative overhead.

Our main production database, on the other hand, I cannot imagine going virtual....

Kendall
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I work with MSSQL and MySQL servers on numerous servers. A couple years ago I was hesitant to start setting up SQL servers on VMs because I had heard about the performance issues of running a SQL server on a VM. However, I was surprised after I setup my first couple SQL servers and saw no change in performance. More and more of the servers I work on are on VM and almost all of the larger enterprise clients I work for have virtulized SQL servers.

Yes, the VM does add some overhead cost and if you are going to be hosting multiple VMs on a single box you are going to need a nice beefy server. A common resource problem to look out for is adding additional VM's and thinning out the available resources. It's common practice to plan for some growth, but if you bought your server to host 2 or 3 VMs and now its running 10 VMs your probably going to see a performance hit.

I would be lying if I said I have never seen performance issues running a SQL server on a VM. But, I have learned that if you are seeing poor performance, there probably is something wrong with the environment.

Chris
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