I just wondered what type of disk people use for their Hyper V VMs.
Fixed or Dynamically Expanding?
On reading up on it, I can see what the main advantages/disadvantages are.
More efficient use of disk space in the area used to store the VHDs
As disk expands the VHD file can become fragmented on the disk used to store it and
the disk IO on the VM can be adversely affected, if it keeps having to expand.
Once the pre-configured "max size" is reached you cannot seem to expand it further?
But the main issue I have with the fixed disk type is the space it uses when backing up the VM (at the host level) to DPM 2010.
For the VMs that we have setup with a fixed disk the space allocated for the replica on the DPM storage pool is the full size of the VHD file (e.g. if you have allocated 200GB as a fixed disk DPM will backup the entire file).
The first replica can therefore take ages, but after that only changes to the VHD are replicated. It is quite inefficient use of D2D backup disk space but that's not a main problem as disk space is cheap and I'd rather backup at the host level rather than Guest so we can restore the entire VM in a disaster recovery scenario.
We use D2D2T and the problem is that backing up all of our physical and virtual servers is just not quite fitting onto one tape. We have an LTO5 autochanger (1.5TB Uncompressed/3TB Compressed) and it's backing up 2.9TB of data, running of space and and then backing up the remaining 80GB to another spare tape.
Good compression... which I am assuming is because it's compressing the unused part of the VHD file very well (because it's all zeros!).
There is also the matter of whether to convert the disk type within the Guest OS Disk Management to Basic or Dynamic; we've kept them as Basic because I found out from someone who posts to these forums that having any Dynamic disks on a Guest OS will cause DPM to backup the VM in Saved state mode (the VM goes offline while it is being backed up). It is listed in one of Microsoft's articles as a requirement for doing online backups of VMs, but they should really be more open about this.
The VMs that I am referring to are file servers so I would prefer the fixed disk type for performane.
Any advice, experiences appreciated.
Last edited by Bruce123; 29th November 2011 at 08:49 PM.
Can't answer all your questions about backing up VMs using DPM, but some significant improvements have been made to the performance of dynamically expanding VHDs in Server 2008 R2. In fact so much so that I believe dynamically expnding VHDs are now considered perfectly adequate for all but the heaviest workloads.
We decided to go for dynamically expanding VHDs for our server VMs when we started to virtualise last year and I've not seen any performance issues that would indicate a problem with this. There is a very interesting whitepaper availbale from Microsoft - here - that goes into a lot more detail and compares performance of dynamically expanding VHDs between server 2008 and r2
Also according to this article, you can expand the max size of a dynamically expanding VHD. Never tried it though.
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