The merit of multiple virtual harddrives

[drewdat]

When setting up a new VM is there any merit to adding multiple virtual hard drives (e.g. one for OS files, and another for application/data files) rather than creating one large virtual hard drive?  I've always setup physical servers with one set of disks for the OS files and another set of disks with the data files for IO paging and redundancy reasons.  With virtual hard drives do these advantages still apply?

[vmroyale]

Hello.

Best practice is the same in the virtual machine as it is in the physical - Separate volumes for OS and data.  Having different VMDKs also allows you greater flexibility later on.  I don't usually separate the pagefile volume out, but there could be valid reasons to do that as well.

Good Luck!

[RaymondG]

I only separate OS and data if it is for a server that requires a lot of data like a database.  If the os and data needs do not exceed 50GB or so I make it one disk.   I've havent had any issues so far.  I honestly dont see the point in separating unless a new drive was requested after the initial VM was creates, or you plan to have the data on slower cheaper disks.

[AndreTheGiant]

Also on physical system you want to use different volumes for different type of data.

In a virtual environment there can more reason to make more volumes (and each volume = 1 vmdk):

• vmdk can be extended... so also volume can be extend if guest OS support it
  
• vmdk can be put on different type of LUN
  
• some backup can take only some vmdk
  

Andre

[shah_boy · Accepted solution]

Here's some reasons I used separate disks instead of partitions or one big Disks

• Management
  
  • Easier to grow the disks
    
    • if all partitions are on the same disk it's more difficult to add space.
      
      • Windows 2008 supports this on the fly for ESX 4.x (and probably other OSes)
        
  • If your Datastore fills up (It happens when you least expect it. usually caused by another admin while you were on vacation) it's easier and quicker to relocate the small disks to get you out of trouble quicker
    
• Performance
  
  • You might want to move your more high IO workloads to different LUNs or different type of Storage NFS/iSCSI/fibre and leave your OS partitions on different types of Storage
    
• Backup
  
  • You might only want to backup the OS partition or the data partition
    
  • Restore you might only want to restore the OS or the data partition
    
  • Replication You might only want to replicate the Data partition
    
• Architecture
  
  • To get better usage and performance out of NFS and data dedup like on a NetApp, There are some suggestions that some types of data (OS/PageFile/Application be put in there own volumes) be on the Netapp. http://media.netapp.com/documents/tr-3428.pdf
    

Reason Not to:

•      Disaster:
  

     If for some reason your vmx file becomes corrupted or is missing. It may make finding all the disks that make up your VMs difficult and understanding what order they need to go back together. (if you have a back up the vmx file and the VMs this isn't a problem)

• SRM. Typically LUNs are replicated for SRM but I have not implemented SRM and this may have changed since I haven't really had looked at it for while. If you distribute you vmdk files on different LUNs then this may not work for SRM.
  

• Space if you thick provision then you would probably get better utilization out of the space in a larger vmdk file then smaller ones. If you Thin provision this isn't really much of a factor.
  

Typically I Create VMs with an OS partition and a Data partition (aligned) on the same datastore. If there's reason to move them I do. Mostly it's the simplicity of expanding storage in the VM. If multiple Disks are required ( SQL, Temp/logs/backup/Data) I just create individual disks for each. Even though you have multiple disks you can manage them all from the VM level.

There's very little downside to multiple disks

Hope that helps

-Rich

[vmroyale]

Another downside to a single volume (or VMDK) approach is the issue of some errant app or logging filling up your OS volume.

[ChrisDearden]

If you are going to create seperate volumes , its worth considering putting them on seperate vSCSI controllers - you can then use the paravirtualised one ( for the non System partition ) and get a little bit of "free" performance.