nexoniarz/SchOS
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SchOS/projects/schfs-linux/README.md
Nexoniarz 26bca020b6 licence: SchOS is GPL-2.0-or-later, no longer Apache-2.0 
Nexoniarz is the sole author and SchOS was never published under
Apache-2.0, so this is a straight relicence. LICENSE is the FSF GPLv2 text,
every first-party SPDX header moves to GPL-2.0-or-later, and NOTICE,
README.md, docs/licensing.md and the site say so. projects/schfs-linux
follows the rest of the tree.

The GPL gives copyleft: a fork has to ship its source. It also lifts the
old "Linux drivers, never" rule, since GPL-2.0-only code combines with
GPL-2.0-or-later. "Or later" leaves room for GPLv3, but one binary still
has to pick one version, so docs/licensing.md now keeps Apache-2.0 and
GPLv3 code out of the kernel to keep Linux code possible there. BSD
4-clause code goes from "avoid" to "no", because the advertising clause
is GPL-incompatible.

LDLS still ships no .ko. That boundary is now project policy rather than
a legal requirement, and ldls.md, devices.md and licensing.md section 6
say so. refmod declares license=GPL, and ldls-check passes (27 checks).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-22 08:48:45 +02:00

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SchFS

A copy-on-write filesystem, built from the ground up.

SchFS is a modern-shaped filesystem in the tradition of BeFS, ZFS and btrfs: a copy-on-write B-tree, 64-bit object identifiers, checksummed metadata, and an extent-based layout. It is written from first principles rather than derived from an existing implementation.

Status: early. The on-disk format works and is exercised by a test suite, but it is not stable and there is no compatibility promise yet. Do not put anything you care about on it.

What it is
Copy-on-write B-tree Metadata is never overwritten in place. An update writes new blocks and swings a pointer, so a torn write leaves the previous consistent state rather than a half-updated tree.
Objects, not paths Everything is a 64-bit object id. Directories are a mapping from name to id, which keeps naming a separate concern from storage.
Checksummed metadata CRC32c on every inode and on the superblock, verified on read. Corruption is detected where it happens rather than inferred later from strange behaviour.
Derived free space The free map is computed from the allocation tree rather than stored alongside it. There is no second structure to disagree with the first.
Extents Contiguous ranges rather than per-block pointers, so a large file costs a small number of records.

What it is not, yet

Being honest about this up front, because a filesystem's missing features are the part that matters:

  • No snapshots. The B-tree is copy-on-write, which is most of the machinery, but nothing takes or names a snapshot yet.
  • No journal replay beyond what copy-on-write gives for free.
  • No extended attributes, no sparse-file support, no online repair.
  • No format versioning policy. This is the first thing being fixed — see the roadmap below — because everything else is harder to change without it.

Roadmap

1. An on-disk format version, and an upgrade path. Not optional, and it gets harder every day it is postponed: the moment an image exists that somebody cares about, changing the layout means being able to read the old one. The superblock already carries a version field and a checksum; what is missing is the policy — a documented meaning for a version bump, a refusal to mount something newer than the driver understands rather than misreading it, and an explicit upgrade step.

2. FUSE. The first Linux implementation is a FUSE filesystem, not a kernel module. Three reasons, in order of weight: it can be tested with fsx and xfstests, which are far harsher than any suite written alongside the code; it carries no licence entanglement, since libfuse is LGPL and used across a process boundary; and it can reuse the existing image-building and checking code rather than reimplementing the format a second time and having the two disagree.

3. A kernel module. Only after the format is pinned and FUSE passes xfstests. Writing a kernel filesystem against a format that is still moving is how you end up maintaining two of them.

Licence

GPL-2.0-or-later, like the rest of SchOS. See LICENSE.

That also settles the kernel-module question in advance: a Linux module under a GPL-compatible licence can use EXPORT_SYMBOL_GPL interfaces and does not taint the kernel. The FUSE implementation still comes first, for the reason above.