Hi all, I'm the creator of the Windows Btrfs driver. During the course of development, it's become apparent that for 100% compatibility with NTFS there'd need to be some minor changes to the disk format. Examples: Windows' LZNT1 compression scheme, Windows' encryption scheme, case-sensitivity, arbitrary-length xattrs.
I'm loathe to nab a compat flag bit for these, as they're all relatively minor, and most wouldn't be that useful for Linux. And sod's law says that if I unilaterally grab the next bit, the Linux driver will sooner or later use the same bit for something else. Has anyone ever brought up the idea of per-subvol compat flags? The idea is that if a new feature affects the FS root and nothing else, rather than adding a new compat flag bit, there'd be an entry in the root tree after the ROOT_ITEM, e.g. (0x100, 0x85, 0x2ed1c081232d) for a compat entry, or (0x100, 0x86, 0x85f7583bc4b0) for a readonly compat entry With the third number being an arbitrary identifier for the feature (i.e. like a UUID). If the driver doesn't understand a certain feature, it'd make the subvol readonly or inaccessible, as appropriate. It'd also have to disable certain features, such as balancing, but the rest of the FS would be usable. As I see it, there's several advantages to this approach: * Because the feature identifier is a 64-bit integer rather than a bit, it increases the effective compat flag namespace from 64 bits to 2^64. * It makes out-of-tree development a lot easier, as the increased namespace makes it feasible to distribute patches without worrying about what's going to happen upstream. * It lowers the cost of implementing new features (i.e., you don't have to let users know that new filesystems will only work on Linux 5.x). This would be especially important for encryption, as security means that you'd have to make it easy to add new algorithms when necessary. * It allows for features to be controlled by Linux CONFIG flags, meaning that e.g. embedded devices wouldn't be burdened with a new feature for all time. Does anybody have any thoughts? Mark
