Given nimble's compression is it still important to compact a hyper-v dymanic expanding .vhd files?

[Sean Fahey]

I'm especially curious about this now that 2012r2 hyper-v makes it more difficult to compact dynamically expanding disks.  Will Nimble's volume compression negate the need to compact the .vhd and.vhdx disks?

[ndyer39]

Hi Sean,

The rule with compression is that you want to run it in one place only (ie host side if it's not on the storage side, or storage side of it's not on the host side). I personally would use Nimble compression and negate Windows compression.

[kevin_hsieh]

I am sorry, but how does the latest version of Hyper-V make it more difficult to compress dynamically expanding disk? I am not aware of any changes in dynamically expanding disks other than that the performance penalty for using them has really gone down since the original release.

[Sean Fahey]

Sorry, I should have posted the Technet blog link.  From my experience the only reliable way to compact a .vhd or .vhdx disk in server 2012 and 2012r2 hyper-v is with the Powershell method cited in the blog.  

Compacting a Dynamically Expanding virtual hard disk in Windows Server 2012 - Ask the Core Team - Site Home - TechNet Bl…

[kevin_hsieh · Accepted solution]

Oh, that's actually better explained in the title rather than the text. I think they are two different things, and is nothing new with how Hyper-V handles things. Say you have a 500 GB dynamically expanding VHDX, and you fill it with a 500 GB file with random data which doesn't compress. The VHDX will have expanded to 500 GB, and the Nimble will consume 500 GB of capacity. Now you delete the file inside the VHDX. The VHDX won't get any smaller, but the Nimble should get notified of the file deletion through SCSI UNMAP and return the blocks to the pool and the consumed space will decrease. If you copy the VHDX to another system, you will still need to move 500 GB of data. If you compress the VHDX, the resulting file will be much smaller to copy over the network.

In a similar vein, if you have a 500 static VHDX, it won't consume any space on the Nimble until data is actually written, but if you need to copy it it will still be 500 GB.

[Sean Fahey]

Thanks Kevin.  That explains it well.  Basically don't sweat the compact process unless you are going to move the virtual disk to a new location AND the current file size of the virtual disk is (considerably) smaller then the maximum disk size.