What's in a gigabyte? Aka Disk size limitations in Virtual PC

Well actually, what's in 0.00292969 of a gigabyte to be a little bit more precise (aka 3MB). As you probably guessed, it's Virtual PC/Virtual Server trivia time, but Ben's blog entry from Monday made me sit up an think - is the maximum IDE-connectable VHD size 128GB or 127GB as I previously thought?

If you follow the Virtual PC disk wizard, it says the maximim size of disk is 130557MB

As you'd expect, it won't let you create a disk of 130558MB. So sticking with 130557MB for now and firing up a Virtual Machine and running Disk Administrator (it was an XP SP2 guest VM I was trying this on), the disk shows up as 127.49GB (which would round using no decimal places 127GB). To be a little more accurate, 130557/1024 is approximately :) equal to 127.4970703125GB which is 127GB plus 509MB, otherwise known as 3MB short of 127.5GB. Still with me? Thought not!

Still not done with investigating this, I then went across to Virtual Server 2005 R2 and created a disk of 130558MB and tried to attach it to an IDE bus of a Virtual Machine under Virtual Server. Unfortunately, it gave an error saying that you can't have disks on an IDE bus over 127GB. As we already know, and maybe I could push for the screen to be changed here, it's actually 127.5GB less 3MB. However, that doesn't quite have the same snappiness as 127GB.

Still not quite done, I then went back to Virtual PC and connected the 130558MB disk to the client VM and it let me. It even booted successfully, but wouldn't let me logon - it rebooted at the "Loading Settings" page each time.

Interestingly (maybe that's stretching the use of the word "Interesting"), I then still not quite satisfied booted the client VM into safe mode instead. This did let me log-on, so I went back to the Disk Management snap-in to see what it found.

Curiously, it recognises the disk as 127.49GB still. You can then proceed to initialise the disk and create a partition on it, but the maximum size is 130552MB which means 6MB are AWOL.

I'd kind of lost the will to live at this point as I still hadn't answered the original question - what is the maximum size of IDE attached hard disk under Virtual PC: 127 or 128GB.

Pureists might round down to 127GB, but let's not forget that 3MB short of half a GB you can also have. Next time someone asks me, I'll stick to "3MB short of 127.5GB". Now you know!

Cheers,
John.

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