Merge a restored VM's memory diff with workers - #1058
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ChangesDiff-snapshot merge
Priority: ⬇️ Low Estimated code review effort: 4 (Complex) | ~45 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant SnapshotCreation
participant merge_diff_snapshot
participant merge_planner
participant BaseFile
participant DiffFile
SnapshotCreation->>merge_diff_snapshot: merge snapshot diff
merge_diff_snapshot->>merge_planner: validate inputs and plan writes
merge_planner->>DiffFile: inspect sparse diff runs
merge_diff_snapshot->>BaseFile: apply writes and flush
merge_diff_snapshot-->>SnapshotCreation: return merge statistics
SnapshotCreation->>DiffFile: remove merged diff
Merge Risk: 🔵 Low · up to The new windowed merge can choose larger whole-granule writes even when they would nearly fill the disk. Other VMs or concurrent snapshots on the same store could then run out of space. The fix is a small margin in the fallback check. Durability of the merged snapshot is now handled. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The merge remains isolated to a temporary copy, with bounded worker writes and flushing before publication. No new privilege or cross-VM access was established. Recovery after a captured snapshot fails remains insufficiently demonstrated, so the assessment is not minimal. Retained concerns Security review detailsSecurity Blast Radius
Trust Boundaries and Controls
Resilience and Maintainability Implications
Hardening Proposals
🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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Inline comments:
Review comments at @src/commands/common.rs:
- Around line 565-569: Update merge_diff_snapshot_in to sync base_path with
sync_all after all worker results have been aggregated successfully and before
returning stats; propagate sync failures with context identifying the merged
base.
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- Around line 820-821: Update the free-space check that selects whole-granule
writes in the merge planning flow: require available space to exceed
plan.bytes_written by a reserve, using a fixed margin or a fraction of available
space. Preserve the fallback to one write per run when the reserve is
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`snapshot create` of a VM that was itself restored writes the touched pages to a sparse memory.diff and merges them into a reflink of the base memory.bin. The merge copied each data run with one write, from one thread. On a 128 GiB guest whose diff held 38.1 GiB in 2,328,992 runs it was not done after 24 minutes and the caller's 30 minute limit killed the snapshot (#1057). A single thread waits for one read at a time. On the two files of that snapshot (btrfs, compress-force=zstd, page cache dropped first) one worker merged in 1,646 s, at about 2,400 reads a second, and 16 workers in 323 s. The merge now runs one worker for each 256 MiB or 4,096 writes, up to 16: - A first walk writes nothing and counts the diff's runs, which sets the number of workers. - Each worker takes the next 8 MiB of the file nobody has taken (MERGE_WINDOW), so the workers stay busy wherever the diff's data lies. A run that crosses a multiple of 8 MiB is written in pieces, so the writes are the same whichever worker makes them. - The calling thread is one of the workers: a small merge starts no thread, and a host that refuses a thread still merges. - A worker that fails or panics stops the others before their next write. - The merged file is flushed once, after the last write, as before, and a failed flush fails the snapshot. What is written is unchanged: each run at its own offset, the diff's bytes and no more. Also in create_snapshot_core: - The merged diff is removed. It was published with every diff snapshot, 38.1 GiB in the case above, and nothing reads it. - A failed copy or merge removes the unfinished snapshot directory, as the other failure paths do. - merge_diff_snapshot returns what it did and the "diff merge complete" line logs it: bytes_merged, runs, writes, workers, merge_ms. A failed SEEK_HOLE is now an error. It was read as "data to the end of the file", which lays zeros over the base from there on. The merge refuses a base and a diff of different lengths. Tested (x86_64): make _test-unit FILTER=-E 'test(/^commands::common::/)' Summary [ 0.291s] 55 tests run: 55 passed, 1373 skipped make fmt leaves the tree unchanged make clippy exit 0 make _test-root FILTER=-E 'test(/test_user_snapshot_from_clone_uses_parent/)' Summary [ 23.226s] 1 test run: 1 passed, 1778 skipped That VM test now writes 8 MiB in a restored clone, snapshots it, restores a clone of that snapshot and reads the same bytes back, and fails if the merged diff is still in the snapshot. Its merge, on a 1 GiB guest: bytes_merged=55205888 runs=4784 writes=4786 workers=2 merge_ms=315. The other diff snapshot VM tests need a health check that the machine these ran on cannot pass (#1056); CI runs them. Red first, one mutation of this tree at a time: flush removed: a_merge_flushes_the_base_after_its_last_write_and_fails_when_the_flush_fails no stop before the next write: a_failed_write_stops_the_other_workers_before_their_next_write one worker whatever the work: a_merge_takes_workers_for_what_it_writes_not_for_the_size_of_the_file a_merge_holds_the_same_bytes_at_any_window_with_any_number_of_workers Not tested: a host that refuses a thread, and a failing seek. Not run: arm64, a filesystem other than btrfs, and this commit on the 128 GiB guest (the 323 s above is a script that makes the same writes with 16 workers).
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NOT-A-DEFECT: the review bodies on this pull request carry no finding of their own.
Each inline finding has its answer in its thread. The flush is back and tested (RED-VERIFIED: a_merge_flushes_the_base_after_its_last_write_and_fails_when_the_flush_fails). The findings about the disk space that whole granules take concern code that is no longer part of this pull request: it now writes only the diff's bytes and reads no free space, and those findings are carried as open questions by #1063. The last review of the current head found no major issues.
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snapshot createof a restored VM merges its memory diff with up to 16 workers (#1057).Followed by: #1063, which writes whole 1 MiB granules where a diff's runs are many and small. That part changes how much a merge writes, and its free-space rule is still open, so it is kept out of this pull request.
The Problem
A snapshot of a VM that was itself restored is a diff snapshot. Firecracker writes the touched pages to a sparse
memory.diff, fcvm reflinks the basememory.bin, andmerge_diff_snapshotcopied each data run of the diff into that copy with one write, from one thread.On a 128 GiB guest whose diff held 38.1 GiB in 2,328,992 runs, the merge was not done after 24 minutes and the caller's 30 minute limit killed the snapshot. A single thread waits for one read at a time: on the disk under test that was about 2,400 reads a second.
The Solution
The merge runs one worker for each 256 MiB or 4,096 writes, up to 16.
MERGE_WINDOW), so the workers stay busy wherever in the file the diff's data lies. A run that crosses a multiple of 8 MiB is written in pieces, so the writes are the same whichever worker makes them.What a merge writes is unchanged: each run at its own offset, the diff's bytes and no more.
Also in
create_snapshot_core:merge_diff_snapshotreturns what it did, and thediff merge completeline logs it:bytes_merged,runs,writes,workers,merge_ms.A failed
SEEK_HOLEis now an error. It was read as "data to the end of the file", which lays zeros over the base from there on. A base and a diff of different lengths are refused before anything is written.Measurements
On the two files of the failed snapshot (btrfs,
compress-force=zstd:3, page cache dropped before each run), with a script that makes the same writes:Test Results
One VM test runs on a machine whose network cannot pass a health check (#1056). It now writes 8 MiB in a restored clone, snapshots it, restores a clone of that snapshot and reads the same bytes back, and fails if the merged diff is still in the snapshot:
The other diff snapshot VM tests (
test_diff_snapshot_prestart_full_startup_diff,test_diff_snapshot_cache_hit_fast,test_diff_snapshot_memory_size_valid) need that health check and are left to CI.Red first, one mutation of this tree at a time, each run against the same four tests:
a_failed_write_stops_the_other_workers_before_their_next_writeholds the failing write until the other three workers are each inside a write, and holds those until the stop is raised, so its count does not depend on thread order.a_merge_holds_the_same_bytes_at_any_window_with_any_number_of_workerscompares the merged bytes for five window sizes with 1, 3 and 16 workers, and the bytes the writes held against the diff's.Not tested: a host that refuses a thread, and a failing seek. Not run: arm64, a filesystem other than btrfs, and this commit on the 128 GiB guest.
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