Windows 7 - a lot of compression on backup?

August 27, 2020 12:48 PM

I just made an image of a fairly fresh Windows 7 SP1 disk using the free version of Macrium Reflect. The used space sizes reported by Macrium for the hidden and main partitions are 28.1 MB (or .028 GB) and 61.78 GB respectively, but the image file is only 18,310,396 K (or 18.3 GB). Could there be this much compression in making the image file?

I have made and restored an image before, although I admit that my understanding of Macrium is not yet, shall we say, nuanced. I'm about to mail this backup off as a failsafe (we're about to relocate), so I'd like to be sure I'm doing it right.

Am I probably doing it right?

posted by springo to Computers & Internet (10 answers total)

I feel like maybe that backup wasn't actually completed successfully. I use Winclone to back up my Windows partition on my Mac and the only time I have ever had an image be that small is when something interrupted the backup and I didn't notice.
posted by Kitchen Witch at 12:58 PM on August 27, 2020

Better-than-60% compression on random binaries sure doesn’t sound right. If you run windirstat on both the old and restored systems, do they look approximately the same?
posted by mhoye at 1:04 PM on August 27, 2020 [1 favorite]

Only way to be sure is to restore the backup.
posted by kindall at 1:43 PM on August 27, 2020 [2 favorites]

Only way to be sure is to restore the backup.

...to a temporary directory!

Another option is for the backup software to produce a log of what was backed up and compare that to the actual filesystems. Could be sparse file handling, though probably not enough for all the space savings.
posted by rhizome at 2:00 PM on August 27, 2020

I'm wondering if there is some kind of giant-file-for-virtual-memory thing going on, where the giant file is a bunch of zeros until virtual memory is used. Or something. Does anyone know enough about Windows 7 to speak to that?
posted by amtho at 2:14 PM on August 27, 2020 [2 favorites]

Windows itself compresses pretty well- I used to build images and then capture them with ghost and they were regularly about 8 GB and that included windows updates and the full Microsoft office suite.

also, the windows DVD itself is like 4-6GB and it is able to build the entire OS from it...
posted by noloveforned at 3:39 PM on August 27, 2020

Ditto amtho, not sure about Windows per se, but there are often sorts of large sized files for virtual or memory mapped things that are mostly empty. On *NIX like systems these can be done as sparce file which can seem huge but if you dig down and actually look at the space it actually takes it's really pretty small.
posted by zengargoyle at 5:24 PM on August 27, 2020

Stuff in the winsxs (side by side) folder is hardlinked elsewhere in the windows install. Some software winds up double(or more) counting linked files if its not aware of them. Id double check macriums disk usage info against what windows reports for the drive.

60gb sounds way too high for a fresh install. Even 18 sounds a little high, but not out of the question.
posted by TheAdamist at 5:41 PM on August 27, 2020

18GB sounds about right for an image of a reasonably fresh W7 installation, especially if it's a 64 bit one with more than a small handful of applications already installed in it.

The reported size discrepancy could quite plausibly be due to Windows binaries compressing to about 50%, multiple counting of multiply-linked files in winsxs as TheAdamist mentions, and Macrium Reflect omitting the large pagefile.sys and hiberfil.sys temporary files and probably all the System Restore restore points as well (which might well include Windows's own internal total system restoration image).

But I agree with kindall: the only way to be sure for sure is to exercise the backup against the scenario it's designed for. Get a new blank disk drive and find out if you can restore a booting, working Windows installation onto it from the saved image. Backups that never get tested are not real backups.
posted by flabdablet at 6:09 PM on August 27, 2020 [3 favorites]

As flabdalet says, it skips windows restore, temporary apge files and so on. Checking mine, it's about 65GB when my C drive is 100GB. But then 26GB of the C drive is sytem retore, and 20Gb are page files, so 67GB implies not too much compresison on the rest. You could get Treesize Free and see how much your windows/user/program files are to check if it makes sense. But as others say, the only real way is to test it.

You can mount the image as a drive, and look through it, try opening a few files and partially test that it's working.
posted by Boobus Tuber at 11:39 AM on August 28, 2020