forkvm: clone fork files via FICLONE reflink with sparse-copy fallback#212
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forkvm: clone fork files via FICLONE reflink with sparse-copy fallback#212sjmiller609 wants to merge 2 commits intomainfrom
sjmiller609 wants to merge 2 commits intomainfrom
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CopyGuestDirectory now attempts an FICLONE per regular file before falling back to the existing SEEK_DATA/SEEK_HOLE sparse copy. On btrfs/xfs+reflink the per-fork file copy collapses to a metadata operation, leaving the existing sparse-copy path intact for filesystems that don't support copy-on-write. The reflink path short-circuits after the first observed "unsupported"-class error from FICLONE so we don't repay the kernel rejection on every file. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
The reflink fast path now succeeds before sparse seeking is invoked, so the test must disable reflink to exercise the sparse fallback.
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Summary
forkvm.CopyGuestDirectoryso fork data dirs share extents with their source on supporting filesystems (XFS reflinks, btrfs, ZFS).Why
Snapshot fan-out forks bottleneck on the rootfs copy. FICLONE makes the per-fork copy O(metadata) instead of O(bytes), which is a ~10x speedup on warm caches and removes write amplification on cold ones. Falling back keeps macOS/non-XFS dev environments working.
Test plan
go test ./lib/forkvm/...(unit tests cover both reflink and sparse fallback paths via fault injection)🤖 Generated with Claude Code