Klennet Storage Software

Disk image files vs. physical disk clones

In every cloning project there is a choice between acquiring the image file or making a clone to a physical drive. I will outline advantages and disadvantages of each, in no particular order. You can review these and decide for yourself which way to go, for each particular case you have.

Disk image file

  • Raw image (bitwise copy) is compatible with any data recovery software.
  • Raw image files lack metadata. For example, physical sector size cannot be stored in a raw image.
  • VHD or VHDX images do not store zero data and occupy less storage.
  • VHD and VHDX images are allocated in blocks, so they can't have arbitrary size, their size must be an integral multiple of the block size (2 MB in most cases). Therefore, images may be slightly larger than their source drives.
  • Disk image files can be further compressed (either zip-like or by native filesystem compression) or deduplicated for archival purposes.
  • Disk images are versatile in long-term storage. You can mix and match physical drives, or make RAID setups of these, and put, for example, ten compressed image files onto three physical drives arranged in a RAID5.

Clone on a physical disk

  • Physical clone, if made on the same model/revision hard drive, is as close to the original as you can ever get. The only thing different save for bad block data will be a clone drive serial number. You can try to put it back into whatever original environment and it will often work.
  • Physical clone requires a drive to be the same size as the original. One may think it is safe to use larger drives to hold clones, but that's not true. First and foremost, if you use larger drive, the data past the end of the original must be zeroed so to not contaminate the clone with whatever residual data on the target drive. This is usually achieved by zero-filing the target drive before cloning. Now, use of the larger drive breaks all address calculations and pointers which are computed taking end of the drive as reference. This does not affect common simple partitioning schemes, but often affects RAID metadata.
  • Physical clones occupy more physical space per clone, if you happen to be concerned.
  • If you need redundancy for physical clones, cost increases by the factor of two. Image files can be put on RAIDs at a lower cost.
  • Physical drives must be carefully selected to match sector size with the source or else undesired side effects will occur in subsequent processing.
  • Physical clones are mounted on Windows immediately after cloning completes. If an NTFS volume on the clone has some pending journal entries, these will be replayed. If there are pending SpotFix operations, these will be performed. In general, Windows will touch any compatible filesystem and change it some, immediately once cloning completes. Linux or other incompatible filesystems are not affected. To avoid this, one trick is to first copy the source to image file and then copy the image file to the physical disk. One may argue that whatever changes are to be made to the target on mount are also to be made to the source, so there is no additional changes introduced. This is true but for bad blocks. Source may have bad blocks preventing changes from being made, and the target will likely have no bad blocks.