FSKit questions and clarifications

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:

1. 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.
2. 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.
3. 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.