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schfs: snapshots

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A snapshot is a tree root that is kept. The B-tree is copy-on-write, so
the blocks a committed root reaches are already immutable -- nothing
overwrites a block a committed tree can reach -- and retaining one costs
a record: (SNAP_OBJECTID, KEY_SNAP, id) -> struct schfs_snap.

The record names the root as it was *before* the record was added.
Inserting it rewrites the path under copy-on-write and produces a new
root, so a snapshot that included its own creation would name a block
nobody had written yet.

What makes it survive is the derivation. There is no free-space map on
disk; it is rebuilt at mount by walking the tree, and that walk now
covers every snapshot root as well as the live one. Without it a block
reachable only from a snapshot looks free, gets handed out, and
overwrites the data the snapshot exists to keep -- silently, surfacing
only when the snapshot is read. A snapshot's own tree may hold older
snapshot records; those are deliberately not followed, or deleting a
snapshot could be undone by an older one's memory of it.

Runtime frees are suspended while a snapshot exists. Knowing which freed
blocks a snapshot still needs requires per-extent reference counting,
which is a design and not a flag; until that exists the safe answer is to
free nothing and let the next mount's derivation reclaim what is
genuinely unreachable. Allocation is unaffected and stays correct -- it
works from the in-use bitmap the derivation filled. The running
free-blocks *counter* does drift from the truth while a snapshot is
present, which is cosmetic and is reported by the test rather than
asserted away.

Tested in schfs_objtest: after a snapshot, the live tree shows new
contents while the snapshot still shows the old ones; a file deleted from
the live tree is still readable through the snapshot; deleting the
snapshot removes the record and the space comes back at the next mount.

schfs-check passes including the round-trip; bootcheck 108 [ok].

Not yet: snapshots are not reachable by name from userspace -- there is
no mount-a-snapshot path, so this is the format and the mechanism, not
the feature a user sees.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Nexoniarz 2026-08-10 19:10:24 +02:00
commit 6e2e9e8d05
6 changed files with 333 additions and 5 deletions
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70
kernel/fs/schfs/objw.c
View file

@ -42,6 +42,76 @@ schfs_alloc_objectid(struct schfs_vol *v)
return v->next_objectid++;
}
/* ------------------------------------------------------------------ */
/* Snapshots */
/* */
/* A snapshot is a tree root that is kept. Copy-on-write means the */
/* blocks that root reaches are already immutable -- nothing overwrites */
/* a block a committed tree can reach -- so retaining one costs a */
/* record, and keeping it alive is entirely schfs_space_scan's job. */
/* ------------------------------------------------------------------ */
// Take a snapshot of the volume as it is committed *now*.
//
// The root recorded is the one in v->tree before this call, which is the
// committed state. It cannot be the root this call produces: adding the record
// rewrites the path to it under copy-on-write, so that root does not exist
// until afterwards, and a snapshot naming it would name a block nobody had
// written yet.
//
// The caller commits. Until it does, the record is not on disk and the
// snapshot does not exist -- which is the correct failure mode, because a
// snapshot that survived a crash while the tree it names did not would be a
// dangling root.
int
schfs_snap_create(struct schfs_vol *v, uint64_t id, const char *name,
unsigned namelen)
{
schfs_cache_invalidate();
if (namelen >= SCHFS_SNAP_NAME_MAX)
return -1;
struct schfs_snap sn;
memset(&sn, 0, sizeof sn);
sn.root = v->tree.root;
sn.root_crc = v->tree.root_crc;
sn.root_generation = v->tree.root_gen;
sn.created = v->tree.generation;
for (unsigned i = 0; i < namelen; i++)
sn.name[i] = name[i];
struct schfs_key k;
schfs_mkkey(&k, SCHFS_SNAP_OBJECTID, SCHFS_KEY_SNAP, id);
int r = schfs_btree_insert(&v->tree, &v->ctx, &k, &sn, sizeof sn);
if (r == 0)
v->space.flags |= SCHFS_SPACE_RETAIN_ALL;
return r;
}
// Forget a snapshot. The blocks it was holding are not freed here: they stop
// being reachable, and the next derivation at mount simply does not mark them.
// That is the whole deletion -- there is no reference count to decrement,
// because there never was one.
int
schfs_snap_delete(struct schfs_vol *v, uint64_t id)
{
schfs_cache_invalidate();
struct schfs_key k;
schfs_mkkey(&k, SCHFS_SNAP_OBJECTID, SCHFS_KEY_SNAP, id);
int r = schfs_btree_delete(&v->tree, &v->ctx, &k);
if (r != 0)
return r;
// Stop retaining. If other snapshots remain, the next mount's derivation
// sets the flag again -- it is the walk that knows, not this function, and
// guessing here would mean keeping a second count to disagree with it.
// Until that mount the allocator is merely conservative, which is the safe
// direction.
v->space.flags &= ~(unsigned)SCHFS_SPACE_RETAIN_ALL;
return 0;
}
int
schfs_inode_put(struct schfs_vol *v, uint64_t ino, const struct schfs_inode *in)
{