SchOS/docs/architecture/schfs.md

SchFS — design

Status: phases 1 and 2 are done. The copy-on-write B-tree, the volume layer, the object layer, a working image writer and a checker all exist, and the kernel mounts a SchFS volume read-only at boot. Phase 3 (writes and transaction groups) is next. See §8 for exactly what is built and how to run its tests.

Supersedes the on-disk format described in include/fs.h. Keeps the name SchFS; the on-disk magic changes so v1 images are detected and rejected rather than silently misread.

The brief was "the best aspects of BeFS, ZFS and btrfs." Those three disagree with each other in places, so this document says explicitly what is taken from each and what is deliberately left behind.

1. What each one is actually good at

ZFS — never be inconsistent. Copy-on-write: no block that is part of the live tree is ever overwritten. An update writes new blocks, then atomically flips a single root pointer. A crash therefore lands on either the old tree or the new one — never a half-written mix. Consequence: no fsck, ever. Paired with end-to-end checksums where a block's checksum is stored in its parent pointer, not in the block itself, which makes the whole tree self-validating and catches misdirected writes and bit rot that a self-stored checksum cannot.

btrfs — one data structure, reused everywhere. btrfs has essentially one engine: a copy-on-write B-tree. Inodes, directory entries, extent maps, free space, and snapshots are all just different key types in that same tree. For a small team this is the single most important idea in the whole design: you write and debug one tricky data structure, then get five subsystems from it. Also takes extents with reference counts, which is what makes snapshots and reflink copies fall out almost for free.

BeFS — the part that feels like a database, at a size one person can build. BeFS is a 64-bit journaled filesystem whose distinguishing feature is B+tree indexes over file attributes, with a query interface on top. That is the "SQL-like" capability, and unlike WinFS it actually shipped. Critically, BeFS is also the right scale model: a small team, a readable codebase, and a book (Giampaolo, Practical File System Design, free online) that describes it end to end.

2. What is deliberately not taken

• No ZFS storage pools / RAID-Z / vdevs. Multi-disk redundancy is a large subsystem that solves a problem SchOS does not have yet.
• No btrfs-style rebalancing or online format conversion. Enormous complexity, and it is the historical source of btrfs's worst bugs.
• No actual SQL. No parser, no query planner, no relational engine. See §6 — the useful part of "database filesystem" is the index, not the language. WinFS spent 2003-2006 and a large team on the other interpretation and shipped nothing.
• No deduplication. ZFS's dedup is famous for needing more RAM than anyone expects. Extent refcounts give reflink/snapshot benefits without it.

3. On-disk shape

+--------------------+  block 0        boot block (untouched by the FS)
+--------------------+  blocks 1..N    uberblock array (128 slots, rotated)
+--------------------+
|  CoW B-tree        |  one tree, many key types:
|                    |    (ino, INODE,  0)      -> inode record
|                    |    (ino, DIRENT, hash)   -> directory entry
|                    |    (ino, EXTENT, offset) -> extent mapping
|                    |    (ino, ATTR,   nameid) -> extended attribute
|                    |    (idx, INDEX,  key)    -> attribute index entry
+--------------------+
|  free space        |  extent-based, tracked as tree keys, not a bitmap
+--------------------+

Uberblock array (from ZFS). 128 slots written round-robin, each carrying a transaction number and a checksum. Mount scans all slots and takes the highest-numbered one that validates. This is what makes crash recovery "pick the newest good root" instead of "replay a journal."

Everything is one B-tree (from btrfs). Keyed by (objectid, type, offset), 24 bytes. Adding a feature usually means adding a key type, not a new on-disk structure.

Checksums live in the parent (from ZFS). Each internal node stores, for every child pointer, that child's CRC32c. A block that is read is verified against the pointer that led to it. v1 already chose CRC32c and already has checksum fields in dinode and superblock — same algorithm, better placement.

Widths — every v1 limit that has to die

v1	limit	v2
dirent.inum is ushort	65,535 files, total	uint64 object ids
dinode.size is uint	4 GB max file	uint64
uint block numbers, BSIZE 1024	4 TB volume	uint64 blocks, 4096-byte
DIRSIZ 14	14-char filenames	variable, up to 255
fixed hash table per dir	every dir ≥113 KB, capped at 119 KB	B+tree, pay per entry
addrs[NDIRECT+3]	block-at-a-time	extents with refcounts

Block size moves 1024 → 4096 to match modern media and make extents worthwhile.

4. What already exists and gets reused

Not starting from zero:

• struct schfs_extent is already defined in include/fs.h and unused. The extent concept is already sketched; v2 gives it refcounts and 64-bit fields.
• CRC32c checksum fields already exist in dinode and superblock, and mkfs already computes them. Same algorithm, relocated into parent pointers.
• kernel/fs/log.c is a working transaction log with begin_op/end_op call sites throughout the FS. Under CoW the log is not needed for consistency, but begin_op/end_op are exactly the right places to bracket a transaction group, so the call sites survive even though the implementation is replaced.
• The buffer cache (bio.c) is format-agnostic and needs no change beyond the block-size constant.

5. Phasing

Each phase leaves a bootable system. Do not start phase 2 before phase 1 is solid — the B-tree is the load-bearing piece and everything else assumes it.

Phase 1 — the B-tree, in userland first. Write the CoW B-tree as a plain C library with a block-device abstraction, and test it on a host file with a fuzz/torture harness (random insert/delete/lookup, simulated crash at every write point, verify the tree always mounts). This is the one component where a subtle bug is unfixable later, and testing it in the kernel is far harder. It is also portable straight into tools/mkfs.

Phase 2 — read-only mount. DONE. New magic, uberblock scan, tree walk, inode and dirent key types. tools/mkschfs writes images; the kernel only reads them. The system boots from a ramdisk root (see docs/layout.md and the ramdisk work) so a broken disk cannot stop you from booting.

Went further than planned in two places, both cheaply: data blocks carry checksums too (a CSUM key type, as btrfs does — without it "end-to-end checksums" would cover only metadata, which is the part least likely to rot), and extents landed early because reserving a file's blocks in one go was simpler than not doing it.

Not yet done: SchFS files are not reachable through open(2). The mount is observable through /proc/schfs and through the kernel API in schfs_int.h, but wiring it into namei/readi means the VFS work that phase 3 needs anyway.

Phase 3 — writes, transaction groups. DONE. Every schfs_k* call that changes anything is one transaction group, committed by writing a new uberblock before it returns. The full write API — create, link, unlink, rename, write at an arbitrary offset, truncate — lives in objw.c and is exercised by tools/schfs_objtest.c.

Two things went differently from the plan above, both for reasons worth recording.

Free space is not tracked as tree keys. Allocating from a free map that lives in the tree mutates the tree, which allocates, which changes the map; btrfs resolves that with a fixpoint loop and ZFS with log-structured space maps. SchFS sidesteps it: the committed tree already says exactly which blocks are live, so the map is derived by walking the tree once at mount and held only in RAM. It cannot disagree with the filesystem, because it is computed from it, and a crash raises no question at all. The cost is mount time proportional to metadata size, and a ceiling on volume size set by how much bitmap fits in memory. If mount time ever hurts, the map can be cached on disk purely as a hint, validated against the generation and discarded on a mismatch — an addition, not a format change. See kernel/fs/schfs/space.c.

Data blocks needed a generation stamp too. The B-tree already rewrites its own nodes in place within a group, since a block that no committed root can reach is nobody's fallback. Data blocks had no such marker, so every hundred-byte write copied a whole 4 KB block and split an extent record around it. schfs_extent gained a 64-bit gen, and the same rule now applies to file data.

Phase 3½ — reachable through open(2). DONE. SchFS is a mounted filesystem:

mkdir /mnt
mount -t schfs /dev/hdb /mnt
ls /mnt ; echo hi > /mnt/tmp/w ; cat /mnt/tmp/w ; mkdir /mnt/tmp/d

The VFS grew a real dispatch layer rather than a set of if (dev == SCHFS_DEV) checks. struct fs_ops (include/file.h) is one table per filesystem, chosen by iget() from the device number and hung off the inode; v1's implementations keep their bodies and are simply named v1_*. Adding a third filesystem is a new table and nothing else. kernel/fs/schfs/vfs.c is SchFS's, and it is pure translation — object ids in, inodes out — because all the filesystem logic already lives below it.

Three things had to change on the way, none of them SchFS's fault:

• struct inode.inum and .size were 32-bit, capping objects at 2³² and files at 4 GiB on a filesystem that promises far more. Both are uint64 now, as are readi/writei offsets and struct file.off.
• struct dirent carries a 16-bit inum and a 14-byte name, so it cannot represent a SchFS entry at all. getdents64 is now an fs_op: each filesystem emits the record stream itself, and f->off is whatever position it needs to resume. v1 walks its hash buckets; SchFS walks a key range and synthesises "." and "..", which it does not store.
• Mounting is a substitution made during path resolution (mount_add / mount_cross in fs.c). Nothing on either disk records it. The reverse direction matters as much as the forward one: .. at a mounted root has to leave the mount, and the filesystem underneath has no idea it is mounted anywhere.

Three long-standing bugs surfaced the moment real commands ran against it, all in the shared VFS rather than in SchFS:

• include/fcntl.h used xv6's open flags while every program in the userland sends Linux's. O_APPEND (0x400) landed on O_TRUNC, so cmd >> file truncated it; O_TRUNC (0x200) landed on O_CREATE, so cmd > file left whatever was past the new end in place. Nothing about either pointed at a header.
• sys_openat silently dropped O_TRUNC and O_APPEND while sys_open honoured both, so the same redirection behaved differently depending on which entry point libc happened to call.
• rmdir had no Linux syscall number, so every Linux-ABI program got ENOSYS.

Phase 4 — extents with refcounts. Enables reflink copies and snapshots nearly for free.

Phase 5 — attribute index and query. See below.

6. The "SQL" part, honestly

What you want from "works like SQL" is: ask a question about a lot of files and get an answer fast, without walking every directory. That is an index, not a query language. BeFS's design:

• Files carry typed attributes (string, int, time, …).
• An attribute can be indexed: a dedicated B+tree keyed by attribute value, mapping value → object ids.
• A query is a predicate over indexed attributes, evaluated by intersecting index ranges.

In SchFS this is just another key type — (idx, INDEX, key) — in the tree that already exists. That is the whole reason to build phases 1-4 the btrfs way: the "database filesystem" feature becomes a few hundred lines on top of machinery already written and debugged, rather than a separate engine.

A concrete query() syscall and a find-like tool come after that, and are small by comparison.

7. Honest cost

Phases 1-3 are roughly 3,000-4,000 lines and are the bulk of the work. Phase 1 alone (B-tree + torture harness) is worth doing carefully and unhurriedly; every later phase compounds on it.

This is the largest single piece of work in SchOS. It is also entirely ordinary filesystem engineering with excellent published references (Giampaolo for BeFS; the btrfs design papers; the ZFS uberblock/checksum-in- parent scheme) — there is no research risk here, only volume.

Do this before the installer. The installer's entire job is to create and populate this format. Building it against v1 means building it twice.

7½. Limits

Three different things get called "the limit", and they are worth keeping apart: what the on-disk format can express, what this implementation can currently mount, and what the rest of SchOS can carry. The smallest one is what you actually hit.

The format

	limit	where it comes from
Volume	64 ZiB (2⁶⁴ × 4 KiB blocks)	uint64 block numbers
File	16 EiB (2⁶⁴ bytes)	schfs_inode.size
Files per volume	1.8 × 10¹⁹	uint64 object ids
Hard links per file	4.3 × 10⁹	uint32 nlink
Filename	255 bytes	SCHFS_NAME_MAX
Entries per directory	no fixed limit	one tree key per name hash
Extent	64 ZiB in one record	uint64 nblocks
Tree depth	8 levels	SCHFS_MAX_LEVEL
Block size	4096 bytes, fixed	SCHFS_BLKSIZE; a mismatch is refused at mount
Transaction groups	1.8 × 10¹⁹	uint64 generation, never reused

Directory depth is unbounded on disk; path resolution is what limits it.

This implementation

	limit	where it comes from
Volume, mounted	64 GiB	the in-RAM allocation bitmap: SCHFS_SPACE_PAGES (512) pages × 32768 blocks
Volume, via the buffer cache	4 TiB	bio.c addresses blocks with a uint, and one SchFS block spans four of them
RAM per mount	8 KiB per GiB of volume	two bitmaps, one bit per block each
Inline value in a leaf	1016 bytes	SCHFS_MAX_INLINE = a quarter of a node
Colliding names in one directory	~60, sharing a hash	the packed list for one hash must fit in one inline value
Directory entries at 8-byte names	~42 per key, unlimited keys	as above
Records per leaf / children per node	127 / 84	4 KiB minus a 32-byte header
Mount time	one pass over the metadata	the allocation map is derived, not stored

The 64 GiB mount ceiling is the one that bites first, and it is one constant: raising SCHFS_SPACE_PAGES to 4096 makes it 512 GiB at 64 KiB of pointers per mount. It is set where it is because the page-pointer vector lives inside struct schfs_vol, which the kernel holds as static storage.

The rest of SchOS

These are VFS and driver limits rather than SchFS ones:

• MAXPATH caps a path at 128 bytes.
• NMOUNT allows 8 mounts.
• The IDE driver is 28-bit LBA: 128 GiB per disk.
• A single read/write call is capped at 4 GiB (n is a uint); the file itself is not.

The two that used to be here — a 16-bit inode number in struct dirent and a 32-bit inum/size in struct inode — are gone. getdents64 is a per- filesystem operation now, so SchFS emits its own records with 64-bit inode numbers and names up to 255 bytes, and the inode carries 64-bit numbers and sizes throughout.

8. What exists now

include/schfs.h              on-disk format + compile-time layout assertions
kernel/fs/schfs/btree.{c,h}  the copy-on-write B-tree and its cursor
kernel/fs/schfs/schfs_int.h  volume + object layer interface
kernel/fs/schfs/space.c      allocation bitmap, derived from the tree at mount
kernel/fs/schfs/vol.c        uberblock ring: open, format, commit
kernel/fs/schfs/obj.c        read side: inodes, dirents, extents, path lookup
kernel/fs/schfs/objw.c       write side: create, link, unlink, rename, write
kernel/fs/schfs/kmount.c     kernel: block device, mount, transaction bracket
kernel/fs/schfs/vfs.c        the fs_ops table: SchFS behind open/read/write
tools/mkschfs.c              build an image from a directory tree
tools/schfs_fsck.c           verify, list, extract
tools/schfs_test.c           B-tree torture harness
tools/schfs_objtest.c        object layer: files, directories, space accounting
tools/schfs_roundtrip.sh     end-to-end filesystem test
tools/mutate.sh              mutation-tests the suites themselves
tools/bootcheck.ps1          boots under QEMU and drives the shell

Everything except kmount.c is compiled both into the kernel and into the host tools, so the writer and the reader cannot drift apart: there is one implementation of the format and nothing to disagree with.

Running the tests

make schfs-check         all three suites
make schfs-test          B-tree only
make schfs-objtest       object layer only
make schfs-image-test    round-trip only

make schfs-check after touching anything under kernel/fs/schfs.

The round-trip's oracle is the host's own filesystem: an image is built from a directory tree, extracted with a separate program, and diff -r'd against the original. It also asserts hard-link counts (which diff cannot see), checks that rebuilding produces byte-identical output, and runs the whole thing a second time with the name hash cut to 6 bits so the packed-dirent collision path — which a 64-bit hash would never reach — is actually exercised.

schfs_objtest runs against a memory-backed volume, so a full create/write/delete/remount cycle takes milliseconds. Its most important check is the least glamorous: after deleting everything, free space must be back to exactly what it was on an empty volume. Every other test in the suite still passes on a filesystem that leaks a block on every delete.

tools/mutate.sh injects six deliberate bugs and fails if any of them goes undetected. It is how the suites are kept honest; see the note in §9.

Verifying on real hardware

make ramroot             root from RAM, so device 1 is free
make build/schfs.img
make qemu-schfs          then, at the shell:  cat /proc/schfs

.\run.ps1 -SchFS does the same on Windows, and tools\bootcheck.ps1 -SchFS does it unattended and asserts on the transcript.

9. Things learned the hard way

Recorded because each one cost real time and none is obvious from the code.

• The root's object id must be reserved. next_objectid started at SCHFS_FIRST_OBJECTID, which is the root's id, so the first directory created silently became the root. It then contained itself, and every tree walk recursed until the stack ran out. schfs_fsck --check now detects directory loops explicitly rather than crashing.
• Cursor buffers alias. A range callback gets a pointer into the cursor's block buffer, and almost every readdir caller immediately looks the entry up, which overwrites it. Directory iteration copies each entry out before calling back. Extracting a tree with directories reparented under their own children is what this looks like when it goes wrong.
• A 4096-byte block does not fit on a 4096-byte kernel stack. The cursor exists precisely to avoid one block buffer per tree level, but schfs_vol_open still kept a whole uberblock as a local and blew the stack on the first boot. The kstack canary caught it cleanly.
• Extents only merge if you allocate the run first. Writing an extent record allocates blocks for its own copy-on-write path, so allocating data one block at a time guarantees the blocks are never adjacent and every block needs its own extent. schfs_file_reserve claims the whole run up front; an 822 KB binary is one extent instead of 201.
• Freed blocks are reusable during a build. Copy-on-write forbids reusing a released block because the previously committed tree still points at it — but a build has committed nothing. Recycling them cut the test image from 3596 blocks to 374.
• Reproducibility dies from atime. mkschfs reads every source file, which updates its atime, so copying atime into the image made two builds of the same tree differ. All three timestamps come from mtime.