d0f7acf9eb
An x86-64 operating system built from xv6, now largely rewritten: SMP, a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF rasteriser, and a Linux-numbered syscall ABI. No history before this point -- the tree was never under version control. This commit exists so the rename that follows has something to undo to.
72 lines
3.5 KiB
Markdown
72 lines
3.5 KiB
Markdown
# SchFS
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A copy-on-write filesystem, built from the ground up.
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SchFS is a modern-shaped filesystem in the tradition of BeFS, ZFS and btrfs: a
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copy-on-write B-tree, 64-bit object identifiers, checksummed metadata, and an
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extent-based layout. It is written from first principles rather than derived
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from an existing implementation.
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**Status: early.** The on-disk format works and is exercised by a test suite,
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but it is not stable and there is no compatibility promise yet. Do not put
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anything you care about on it.
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---
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## What it is
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| **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. |
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| **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. |
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| **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. |
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| **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. |
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| **Extents** | Contiguous ranges rather than per-block pointers, so a large file costs a small number of records. |
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## What it is not, yet
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Being honest about this up front, because a filesystem's missing features are
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the part that matters:
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- **No snapshots.** The B-tree is copy-on-write, which is most of the machinery,
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but nothing takes or names a snapshot yet.
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- **No journal replay** beyond what copy-on-write gives for free.
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- **No extended attributes**, no sparse-file support, no online repair.
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- **No format versioning policy.** This is the first thing being fixed — see
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the roadmap below — because everything else is harder to change without it.
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---
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## Roadmap
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**1. An on-disk format version, and an upgrade path.** Not optional, and it gets
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harder every day it is postponed: the moment an image exists that somebody cares
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about, changing the layout means being able to read the old one. The superblock
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already carries a version field and a checksum; what is missing is the policy —
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a documented meaning for a version bump, a refusal to mount something newer than
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the driver understands rather than misreading it, and an explicit upgrade step.
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**2. FUSE.** The first Linux implementation is a FUSE filesystem, not a kernel
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module. Three reasons, in order of weight: it can be tested with `fsx` and
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`xfstests`, which are far harsher than any suite written alongside the code; it
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carries no licence entanglement, since libfuse is LGPL and used across a process
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boundary; and it can reuse the existing image-building and checking code rather
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than reimplementing the format a second time and having the two disagree.
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**3. A kernel module.** Only after the format is pinned and FUSE passes
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`xfstests`. Writing a kernel filesystem against a format that is still moving is
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how you end up maintaining two of them.
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---
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## Licence
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Apache-2.0. See [LICENSE](LICENSE).
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Apache-2.0 rather than MIT or BSD for one reason that matters at this level: it
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contains an express patent grant and a patent-retaliation clause. MIT and BSD
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are silent on patents, which means a contributor can hand you code and later
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assert a patent over it.
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Note that a Linux **kernel module** is a different licensing question from FUSE,
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and one reason the FUSE implementation comes first.
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