I'm looking at the differences between using a file versus a partition to store a virtual disk image in VM use. The common knowledge is that partition-based images are faster than file-based images because of a decreased overhead. It makes sense, but I've never seen any actual numbers.

My own testing bears out a different result. When I benchmark a direct-to-partition virtual disk, then format that same partition with ext4, create a virtual disk image stored on that ext4 filesystem, and then benchmark that, I see no speedup at all for the direct-to-partition virtual disk. Instead on some systems the file-based image is even faster (possibly due to host OS caching or something like that). This test was repeated many times on many systems, with fairly consistent results.

So perhaps throwing out the performance justification, is it still considered better to use a partition rather than a virtual disk image? Is there some other reason why direct partition access is better than image files?

Or perhaps is there some reason to go the other way around? Perhaps an advantage in one of the virtual disk file formats that you don't get with raw partition images?