Known Limitations

Before trying ExtendFS, please know the following:

• Not all ext4 features are supported. The following feature flags are supported: filetype, recover ^[1], extents, 64bit, mmp ^[2], flex_bg, csum_seed ^[3], largedir, and casefold. If your volume requires other incompatible features, like inline_data, meta_bg, compression, or encryption, it won't mount.
• LVM is not currently supported.
• Performance may be lower than it would be with a kernel extension (e.g. FB21069313 ^[4]). A large part of this is FSKit overhead, but there is also likely some room for improvement in this code.
• As stated elsewhere, this is a read-only implementation. You cannot write data to an ext4 volume using ExtendFS.
  • Write-related options such as creating new folders or deleting items might unexpectedly appear in the Finder. This is an FSKit bug (actually, various bugs - FB19241327, FB21068845). Performing those actions does not actually write any data to the disk.
• If there are multiple hard links to the same file in a folder, only one of the hard links appears in the Finder (FB21021220). You can still use command line tools like ls(1) to enumerate the hard links.
• An internal ext2/3/4 volume might not auto-mount, and you may need to use the mount command in Terminal in order to mount it (#2, FB21729650).

FSKit is still very young and Apple is rapidly adding new features and fixing bugs in the framework in macOS updates. I encourage you to use the latest version of macOS possible if you are encountering issues.

[1] ExtendFS currently cannot repair your disk. If it is marked as needing recovery, it will still try to mount, but you may see a warning that the disk could not be repaired. If you see this, you should attempt a repair using fsck on a Linux machine.

[2] MMP does not perform any checks because volumes can currently only be mounted read-only.

[3] ExtendFS only verifies checksums on some data structures at this time.

[4] If you're wondering what the FB numbers are, these are Apple feedback numbers for various bugs I filed against FSKit. You unfortunately can't see them, but they're mostly for my own reference (or to reference by an Apple employee, if they happen to come across this).