Can't compress virtual disk

[Salty6]

I am evaluating Parallels Desktop 8 as a replacement for VMWare Fusion, with a Windows XP guest OS. One of the main advantages I see in Parallels is the built-in compressor technology to keep VM sizes minimized.

The VM imported fine, but I cannot achieve any disk compression.

I have an expanding 30 GB virtual disk, about half of which is used and the other half of which is empty space as reported by XP.

I have run Windows disk repair and defrag and have used Eraser to zero out all that free space. But Parallels always reports that it cannot achieve any compression at all, and when I run it anyway that is indeed what happens.

I have tried to compress the disk both by selecting the "Reclaim Disk Space" menu item under "Virtual Machine" and also by selecting "Compress" under Configure>Hard Disk.

This virtual disk has no snapshots, and Undo Disks is disabled.

Any ideas about how to get virtual disk compression to work?

Thanks.

 

[Tony Carreon]

are you certain the vm file is using the space? XP will always report 30gb, even if the virtual disk file is only 10gb. when you compress it, there should be a dialog box that tells asks you to confirm and gives you the amount of space that is used before and after compression.

in the attached image, you can see:
- windows shows that i have a 64gb drive, of which 41.3gb is free
- the actual vm hdd file is 25.5gb
- if i compress it, it will go down to around 23gb

after compression, windows will continue to show that i have 41.3gb free.

as a side note, i'm not sure what eraser is, but when you say it "zeroes out the free space" that makes me think it actually writes 0 to all of the blocks. if that is the case, that space is no longer actually free, it's in use (there's a '0' there) at which point, parallels will not be able to free the space anymore. if it's the same program i found on sourceforge, then that's your issue.

"Eraser is a secure data removal tool for Windows. It completely removes sensitive data from your hard drive by overwriting it several times with carefully selected patterns." "overwrites" is the operative word there. it doesn't empty the space, it uses it...

 

[Salty6]

Thanks, Tony. Yes, the VM is definitely using the entire 30 GB, as shown in the OS X Finder. Windows shows many gigabytes of free space, yet when I try to compress or reclaim disk space in Parallels, I am told there is no space to reclaim (see attached screenshots).

I can't understand why Parallels is unable to achieve any compression of this VM.

 

[Tony Carreon]

Thanks, Tony. Yes, the VM is definitely using the entire 30 GB, as shown in the OS X Finder.
I can't understand why Parallels is unable to achieve any compression of this VM.

the reason parallels can not compress the space is because the space is actually in use. when you used eraser, you actually wrote data to that block. the vm file size was increased to represent this. windows sees it as free, but in reality there is actually data there so parallels can not remove it from the vm file.

parallels compression is one of the few features that has worked flawlessly for me in all versions of windows - xp, vista, 7 and now 8.

 

[Salty6]

Thanks, Tony, but I don't think Eraser could have anything to do with the inability of Parallels to compress the virtual disk.

First, the problem predated the use of Eraser.

Second, to my knowledge every disk, whether virtual or physical, has every sector filled with data at all times—either a one or a zero at each data location, the only two possibilities. It's empty space if it's not associated with a file. I don't see how Eraser laying down a pseudorandom pattern of ones and zeros in the empty space, replacing the ones and zeros already there, could be a factor here.

It's still a mystery, to me at least, why Parallels says it can't compress or delete disk data to reduce my virtual disk size by even a single byte.

 

[Tony Carreon]

when parallels creates a disk, it only allocates the actual space in use, so if nothing has been written to the disk (on window's side) then the space is not actually used (on the mac side).

the only other thing to check is to make sure the disk is set to "Expanding" disk in the VM configuration. if it is not, then parallels will preallocate all of the space and make your virtual disk the full size specified.

 

[Salty6]

Thanks, Tony. Yes, it's an expanding disk, as shown in one of my screen shots above.

Isn't one of the main features of Parallel's compressor technology exactly to reclaim free space: i.e., virtual disk space that was once used but is no longer associated with files?

Perhaps I should submit this question to Parallels tech support. Something seems quite wrong.