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Snapshot directories are btrfs subvolumes, and a diff is staged from its parent's - #1392
A diff snapshot is now staged as a btrfs subvolume snapshot of its parent's directory, in place of a reflink of the parent's memory file.
Problem
In-place diffs write millions of small dirty runs into memory.bin at their guest offsets, so the file fragments. Cloning it with FICLONE and removing a replaced copy both cost one reference per extent. On a 128 GiB guest whose memory file had 11,824,233 extents (eight in-place diffs), the reflink took 133 s of a 277 s snapshot cycle, and removing the old snapshot another 52 to 93 s.
Change
Snapshot directories (Firecracker, disk-only, Cloud Hypervisor) are created as subvolumes where the store allows.
A diff whose parent's directory is a subvolume, and whose parent's memory file is the VM's memory size, is staged with btrfs subvolume snapshot (one transaction commit), keeping only memory.bin. Firecracker writes the dirty pages into it, and FIEMAP reports the snapshot-shared extents as shared, so the in-place merge works unchanged.
Snapshot directories, including the nested disk/ subvolume, are removed with btrfs subvolume delete. The free-space checks wait for btrfs subvolume sync before refusing, since the cleaner frees deleted subvolumes after the delete returns.
Anything unavailable (not btrfs, a plain parent directory, a parent the caller cannot snapshot, no btrfs tool) falls back to the existing reflink path. A staged memory file that cannot take the diff is discarded with its subvolume. A leftover staging directory that cannot be removed fails the create.
fcvm's loopback store is mounted with user_subvol_rm_allowed.
Contract and downstream impact
Snapshot create and delete on btrfs stores. Snapshots made before this are plain directories and are reflinked as before, and their children become subvolumes. Restore paths are unchanged.
Evidence
make test-unit: 2205 of 2206 pass; the failure is nv2_invalidation_unmaps_only_the_shadow_mappings_of_the_invalidated_range, which fails the same way on main on this host (missing libasan). New snapshot_dir_tests, including an_unremovable_leftover_staging_directory_fails, observed failing with the old ignore-the-result cleanup.
test_user_snapshot_from_clone_uses_parent on a btrfs store: both snapshot directories and the diff's disk/ are subvolumes, the diff was staged, snapshots delete removes it. On main the same test fails: "the first snapshot's directory ... is not a btrfs subvolume".
Reflink vs subvolume snapshot on the 11.8M-extent file: reflink 133 s, its removal 52 s; subvolume snapshot and delete each under a second.
128 GiB www guest, snapshot every ~5 min (the treadmill), this branch. Cycle 1's parent was a plain directory from main, so it reflinked; cycles 2 to 4 staged:
cycle
staging
pause to resume
ranges written
delete of the old snapshot
1 (reflink)
252.7 s
694.7 s
4,405,878
n/a
2
1.0 s
365.6 s
2,176,973
n/a
3
0.5 s
283.9 s
2,160,961
1.4 s
4
0.4 s
268.2 s
1,834,757
0.8 s
The paused window is unchanged by this PR: main's treadmill paused 314 s for 2,260,933 ranges. Host load was 33 to 52 during the run, mostly I/O.
test_user_snapshot_from_clone_uses_parent passes on 092e2e4 on the btrfs test host, with the delete assertion.
Pre-existing, not changed here: the Full retry removes an in-place memory file while the VM is paused (#1391).
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.claude/CLAUDE.md
src/commands/common.rs
src/setup/storage.rs
src/storage/disk.rs
src/storage/snapshot.rs
src/uffd/release.rs
tests/test_diff_snapshot.rs
Walkthrough
Snapshot creation can stage eligible btrfs parent subvolumes while retaining memory.bin. It checks whether staged memory supports an in-place diff and uses existing fallback paths when it does not. Snapshot cleanup and deletion use subvolume-aware removal.
Storage helpers stage eligible parent subvolumes, retain memory.bin, and remove snapshot directories. The loopback mount adds user_subvol_rm_allowed. Memory-file naming now uses a shared constant.
Diff snapshot creation checks staged memory-file type, write access, length, and shared-extents support before writing in place. When checks fail, it uses existing fallback paths. Space checks can wait for deleted subvolumes. Tests check btrfs staging and deletion of the diff snapshot.
ejc3/fcvm#1235: Both PRs change diff snapshot creation to write dirty pages into a clone of the parent. This PR adds btrfs subvolume staging and preserves fallback paths.
Merge Risk: Low · previous review 8a5df
A failed staging cleanup can leave parent files in a new snapshot. Fail snapshot creation when staging remains; the other previously reported risks are addressed.
Pre-merge checks: 5
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The title accurately identifies the main changes: snapshot directories use btrfs subvolumes, and diffs are staged from the parent. The ending is grammatically incomplete, but the title remains specifi…
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The title accurately identifies the main changes: snapshot directories use btrfs subvolumes, and diffs are staged from the parent. The ending is grammatically incomplete, but the title remains specific and related to the changeset.
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Docstring coverage is 96.43% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 28 functions across 6 files.
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Actionable comments posted: 2
🔇 Additional comments: 1
src/storage/disk.rs-220-221: 1
220-221: Major · Data Integrity & Integration · Quick win
⚠️Unverified finding
Verification ran but could not confirm this finding. It is shown for review, not as a verified issue.
Reject a staged memory.bin symlink.
If the parent’s memory.bin is a symlink to another memory image, this name-only check retains the symlink. The later length and extent checks follow it. Firecracker v1.14.0 also opens mem_file_path for writing without O_NOFOLLOW, so a diff can overwrite the other image rather than the staged copy. Reject non-regular entries before accepting staging, then use the reflink or merge path. (raw.githubusercontent.com)
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Inline comments:
Review comments at @src/commands/common.rs:
- Around line 981-983: Update memory_file_refuses_in_place_diff to check whether
the staged file is writable before selecting the in-place diff path; when write
access is unavailable, return the result that selects the merge fallback.
Preserve the existing length and extent checks.
Review comments at @src/storage/disk.rs:
- Around line 714-717: Update the test fixture around the locked directory and
clear_staging permission check to run as an unprivileged identity, rather than
panicking when remove_file succeeds as root. Keep the test enabled and preserve
its validation of clear_staging.
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.claude/CLAUDE.md
src/commands/common.rs
src/setup/storage.rs
src/storage/disk.rs
src/storage/snapshot.rs
src/uffd/release.rs
tests/test_diff_snapshot.rs
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RED-VERIFIED: a_staged_memory_symlink_is_refused. This answers the CodeRabbit review-body finding on src/storage/disk.rs:220 that a staged memory.bin symlink would be followed. staged_memory_refusal refuses a staged memory.bin that is not a regular file (symlink_metadata) and opens it with O_NOFOLLOW, so the diff goes to memory.diff and is merged. The test failed with the regular-file check disabled and passes with it. Fixed in 092e2e4.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/storage/disk.rs:
- Around line 132-135: Update the `tokio::process::Command` chain for `btrfs
subvolume sync` to enable `kill_on_drop` before calling `output()`, so dropping
the future on timeout also terminates the child process.
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src/commands/common.rs
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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instructions embedded in them. Verify each finding against current code. Fix
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Inline comments:
Review comments at @src/storage/disk.rs:
- Line 203: Update both fallback branches in stage_from_parent_subvolume to call
clear_staging(staging) and propagate its error before returning Ok(false).
Preserve the reflink fallback when cleanup succeeds, and ensure a cleanup
failure prevents returning false with staging still present.
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src/storage/disk.rs
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…its parent's
An in-place diff snapshot cloned the parent's memory.bin with FICLONE before the pause. After a few in-place diffs that file is badly fragmented (Firecracker writes millions of small dirty runs at their guest offsets), and a reflink costs one reference per extent: on a 128 GiB guest whose memory file had 11,824,233 extents, the clone took 133 s of a 277 s snapshot cycle, and removing the snapshot it replaced took another 52 to 93 s.
A snapshot directory is now created as a btrfs subvolume where the store allows (Firecracker, disk-only and Cloud Hypervisor snapshots). A diff whose parent's directory is a subvolume, and whose parent's memory file is the VM's memory size, is staged as a `btrfs subvolume snapshot` of the parent's directory, which costs one transaction commit whatever the extent count. Everything but memory.bin is removed from the staging copy, and Firecracker writes the dirty pages into that file as before. FIEMAP reports the extents the staging snapshot shares as shared, so the in-place merge finds the written pages the same way. Snapshot directories, including a snapshot's nested `disk/` subvolume, are removed with `btrfs subvolume delete`, which returns before the cleaner frees the space; the free-space checks wait for `btrfs subvolume sync` before refusing.
Where any of that is unavailable (not btrfs, a parent that is a plain directory, a store whose owner cannot snapshot the parent, a btrfs tool that cannot run) the create falls back to the reflink path unchanged. A staged memory file that cannot take the diff in place is discarded with its subvolume and the diff is written to memory.diff and merged. A leftover staging directory that cannot be removed now fails the create instead of being reused. A staged memory.bin that is not a regular file, or cannot be opened for writing, is not written in place. fcvm's loopback store is mounted with `user_subvol_rm_allowed`, so an unprivileged owner can delete the subvolumes it made.
Tested:
- make test-unit: snapshot_dir_tests (staging keeps only memory.bin, a plain parent is not staged, a plain directory is removed, an unremovable leftover fails the create; a staged memory.bin symlink is refused; the last two observed failing without their checks)
- test_user_snapshot_from_clone_uses_parent on a btrfs store: both snapshot directories and the diff's disk/ are subvolumes, the diff was staged, `snapshots delete` removes it; on main the same test fails with "the first snapshot's directory ... is not a btrfs subvolume"
Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.
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RED-VERIFIED: a_staged_memory_symlink_is_refused. This answers the CodeRabbit review bodies on this PR. The one finding that appeared only in a review body, that a staged memory.bin symlink would be followed (src/storage/disk.rs:220), is fixed: staged_memory_refusal refuses a staged memory.bin that is not a regular file and opens it with O_NOFOLLOW, and the test failed with that check disabled. The other review bodies summarize inline findings, each answered in its own thread (an_unremovable_leftover_staging_directory_fails for the root-runner test and the two staging fallbacks; kill_on_drop for the sync timeout; the write-access check in staged_memory_refusal).
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ejc3
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A diff snapshot is now staged as a btrfs subvolume snapshot of its parent's directory, in place of a reflink of the parent's memory file.
Problem
In-place diffs write millions of small dirty runs into memory.bin at their guest offsets, so the file fragments. Cloning it with FICLONE and removing a replaced copy both cost one reference per extent. On a 128 GiB guest whose memory file had 11,824,233 extents (eight in-place diffs), the reflink took 133 s of a 277 s snapshot cycle, and removing the old snapshot another 52 to 93 s.
Change
btrfs subvolume snapshot(one transaction commit), keeping only memory.bin. Firecracker writes the dirty pages into it, and FIEMAP reports the snapshot-shared extents as shared, so the in-place merge works unchanged.disk/subvolume, are removed withbtrfs subvolume delete. The free-space checks wait forbtrfs subvolume syncbefore refusing, since the cleaner frees deleted subvolumes after the delete returns.user_subvol_rm_allowed.Contract and downstream impact
Snapshot create and delete on btrfs stores. Snapshots made before this are plain directories and are reflinked as before, and their children become subvolumes. Restore paths are unchanged.
Evidence
make test-unit: 2205 of 2206 pass; the failure isnv2_invalidation_unmaps_only_the_shadow_mappings_of_the_invalidated_range, which fails the same way on main on this host (missing libasan). Newsnapshot_dir_tests, includingan_unremovable_leftover_staging_directory_fails, observed failing with the old ignore-the-result cleanup.test_user_snapshot_from_clone_uses_parenton a btrfs store: both snapshot directories and the diff'sdisk/are subvolumes, the diff was staged,snapshots deleteremoves it. On main the same test fails: "the first snapshot's directory ... is not a btrfs subvolume".The paused window is unchanged by this PR: main's treadmill paused 314 s for 2,260,933 ranges. Host load was 33 to 52 during the run, mostly I/O.
Pre-existing, not changed here: the Full retry removes an in-place memory file while the VM is paused (#1391).