Hello, I'm playing around with the FSKit API and I'm looking for programmatic ways to mount a file system backed by an FSKit extension. Currently, I could only find the new -F option in the mount command but I would prefer if I could avoid executing comman line tools. I checked the DiskArbitration framework but couldn't find anything specific for FSKit there. Thanks

I reported this as a bug (FB18614667), but also wanted to ask here in case this is actually just me doing something wrong, or maybe I'm misunderstanding the entire use case of metadataRead. (My understanding is that metadataRead is basically read but it checks a cache that the kernel manages before trying to read the physical resource, and in the case of a cache miss it would just go to the physical resource and then add the bytes to the cache. Is that right?) I’m encountering an issue in an FSKit file system extension where (for example) read(into: buf, startingAt: 0, length: Int(physicalBlockSize)) works, but metadataRead(into: buf, startingAt: 0, length: Int(physicalBlockSize)) throws an EIO error (Input/output error) no matter what I do. (Note: physicalBlockSize is 512 in this example.) The documentation (https://developer.apple.com/documentation/fskit/fsblockdeviceresource/metadataread(into:startingat:length:)) indicates that the restrictions on metadataRead are that the operations must be sector-addressed (which is the case here, especially as regular read has the same restriction and succeeds) and that partial reading of metadata is not supported. (I don’t think that applies here?) In a sample project I was able to replicate this behavior where the module only ever reads the block device in its enumerateDirectory implementation, and so trying to list the contents of a directory leads to an "Input/output error" when e.g. running ls on the volume. The enumerateDirectory sample implementation is like so: func enumerateDirectory(_ directory: FSItem, startingAt cookie: FSDirectoryCookie, verifier: FSDirectoryVerifier, attributes: FSItem.GetAttributesRequest?, packer: FSDirectoryEntryPacker) async throws -> FSDirectoryVerifier { let buf = UnsafeMutableRawBufferPointer.allocate(byteCount: Int(blockDevice.physicalBlockSize), alignment: 1) defer { buf.deallocate() } // metadataRead will throw... try blockDevice.metadataRead(into: buf, startingAt: 0, length: Int(blockDevice.physicalBlockSize)) // but read will work. // try await blockDevice.read(into: buf, startingAt: 0, length: Int(blockDevice.physicalBlockSize)) // ... return dummy file here (won't reach this point because metadataRead throws) } I'm observing this behavior on both macOS 15.5 (24F74) and macOS 15.6 beta 3 (24G5074c). Has anyone been able to get metadataRead to work? I see it used in Apple's msdos FSKit implementation so it seems like it has to work at some level.

I've faced with some performance issues developing my readonly filesystem using fskit. For below screenshot: enumerateDirectory returns two hardcoded items, compiled with release config 3000 readdirsync are done from nodejs. macos 15.5 (24F74) I see that getdirentries syscall takes avg 121us. Because all other variables are minimised, it seems like it's fskit<->kernel overhead. This itself seems like a big number. I need to compare it with fuse though to be sure. But what fuse has and fskit seams don't (I checked every page in fskit docs) is kernel caching. Fuse supports: caching lookups (entry_timeout) negative lookups (entry_timeout) attributes (attr_timeout) readdir (via opendir cache_readdir and keep_cache) read and write ops but thats another topic. And afaik it works for both readonly and read-write file systems, because kernel can assume (if client is providing this) that cache is valid until kernel do write operations on corresponding inodes (create, setattr, write, etc). Questions are: is 100+us reasonable overhead for fskit? is there any way to do caching by kernel. If not currently, any plans to implement? Also, additional performance optimisation could be done by providing lower level api when we can operate with raw inodes (Uint64), this will eliminate overhead from storing, removing and retrieving FSItems in hashmap.

I work on EdenFS, an open-source Virtual Filesystem that runs on macOS, Linux, and Windows. My team is very interested in using FSKit as the basis for EdenFS on macOS, but have found the documentation to be lacking and contains some mixed messaging on the future of FSKit. Below are a few questions that don’t seem to be fully covered by the current documentation: Does FSKit support process attribution? Each FUSE request provides a requester Process ID (and other information) through the fuse_in_header structure. Does FSKit pass similar information along for each request? Does the reclaimItem API function similarly to FUSE’s forget operation? If not, what are the differences? See #1 below for why forget/reclaimItem matters to us. Is Apple committed to releasing and supporting FSKit? Is there any timeline for release that we can plan around? Does FSKit have known performance/scalability limitations? We provide alternative methods that clients can use to make bulk requests to EdenFS, but some clients will necessarily be unable to use those and stress the default filesystem APIs. Throughput (on the order of tens of thousands of filesystem requests per minute) and request size are the main concerns, followed closely by directory size restrictions. Why we’re interested in FSKit As mentioned above, my team supports EdenFS on 3 platforms. On Linux, we utilize FUSE; on Windows, we utilize ProjectedFS; and on macOS, we’ve utilized a few different solutions in the past. We first utilized the macFUSE kext, which was great while it lasted. Due to (understandable) changes in supporting kernel extensions, we were forced to move to NFS version 3. NFS has been lackluster in comparison (and our initial investigations show that NFS version 4(.2) would be similar). We have had numerous scalability and reliability issues, some listed below: NFS does not provide a forget API similar to FUSE. EdenFS is forced to remember all file handles that have been loaded because the kernel never informs us when all references to that file handle have been dropped. We can hackily infer that a file handle should never be referenced again in some cases, but a large number of file handles end up being remembered forever. Many of our algorithms scale with the number of file handles that Eden has to consider, and therefore performance issues are inevitable after some time. NFS does not provide information about clients (requesters). We cannot tell which processes are sending EdenFS requests. This attribution is important due to issue #1. We are forced to work with tool owners to modify their applications to be VFS-friendly. If we can’t track down which tools are behaving poorly, they will continue to load excess file handles and cause performance issues. NFS “Server connections interrupted:” dialog during heavy load. Under heavy load, either EdenFS or system-wide, our users experience this dialog pop-up and are confused as to how they should respond (Ignore or Disconnect All). They become blocked in their work, and will be further blocked if they click “Disconnect All” as that unmounts their EdenFS mount. This forces them to restart EdenFS or reboot their laptop to remediate the issue. The above issues make us extremely motivated to use FSKit and partner with Apple to flesh out the final version of the FSKit API. Our use case likely mirrors what other user-space filesystems will be looking for in the FSKit API (albeit at a larger scale than most), and we’re willing to collaborate to work out any issues in the current FSKit offerings.

Hi, I'm working with the new FSKit framework and have successfully implemented a custom file system using FSUnaryFileSystem. Mounting the file system via Terminal works perfectly — I can create, delete, and browse files and directories as expected. Since /Volumes is protected on modern macOS systems, I cannot mount my file system there directly. Instead, I mount it into a different writable directory (e.g., /tmp/MyFS) and then create a symbolic link to it in a user-visible location such as ~/Downloads/MyFS. Finder does see the symlink and displays it with a "Volume" icon, but clicking it results in an error — it cannot be opened. It seems like Finder does not treat the symlinked mount as a fully functional volume. Is there a proper way to register or announce a FSKit-mounted file system so that Finder lists it as a real volume and allows access to it? Are there additional steps (APIs, notifications, entitlements, or Info.plist keys) required to integrate with Finder? Any insight would be greatly appreciated. Thanks!