On Sun, Oct 14, 2018 at 6:20 AM, Cerem Cem ASLAN <cerem...@ceremcem.net> wrote: > I'm not sure I could fully understand the desired achievement but it > sounds like (or this would be an example of selective perception) it's > somehow related with "creating reproducible snapshots" > (https://unix.stackexchange.com/q/462451/65781), no?
No the idea is to be able to consistently reproduce a distro installer image (like an ISO file) with the same hash. Inside the ISO image, is typically a root.img or squash.img which itself contains a file system like ext4 or squashfs, to act as the system root. And that root.img is the main thing I'm talking about here. There is work to make squashfs deterministic, as well as ext4. And I'm wondering if there are sane ways to constrain Btrfs features to make it likewise deterministic. For example: fallocate -l 5G btrfsroot.img losetup /dev/loop0 btrfsroot.img mkfs.btrfs -m single -d single -rseed --rootdir /tmp/ -T "20181010T1200" --uuidv $X --uuidc $Y --uuidd $Z ... shasum btrfsroot.img And then do it again, and the shasum's should be the same. I realize today it's not that way. And that inode assignment, extent allocation (number, size, locality) are all factors in making Btrfs quickly non-determinstic, and also why I'm assuming this needs to be done in user space. That would be the point of the -rseed flag: set the seed flag, possibly set a compat_ro flag, fix generation/transid to 1, require the use of -T (similar to make_ext4) to set all timestamps to this value, and configurable uuid's for everything that uses uuids, and whatever other constraints are necessary. -- Chris Murphy
