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>
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