mount/manager: set sparse file attribute on Windows before mkfs#12938
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mount/manager: set sparse file attribute on Windows before mkfs#12938djs55 wants to merge 1 commit intocontainerd:mainfrom
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On Windows (NTFS/ReFS), extending a file with Truncate does not create
a sparse file by default. When mkfs.ext4 subsequently writes metadata
at scattered offsets across the file, NTFS must zero-fill all
intervening blocks up to the valid data length before each write. For
large filesystem images this is extremely slow.
Fix this by marking the file as sparse via FSCTL_SET_SPARSE before
calling Truncate. This allows NTFS to leave unwritten regions
unallocated, matching the behavior of macOS/Linux where Truncate
already produces sparse files.
Benchmark results (8 GB file, Windows 11, NTFS):
Without FSCTL_SET_SPARSE (before):
Create + Truncate: 535µs (on-disk: 8192 MB)
Scattered writes: 4.74s (on-disk: 8192 MB)
Total: 4.74s
With FSCTL_SET_SPARSE (after):
Create + Truncate: 526µs (on-disk: 0 MB)
Scattered writes: 15.6ms (on-disk: 1 MB)
Total: 16.1ms
~300x speedup for the file creation + write pattern, and on-disk usage
drops from 8 GB to only the blocks actually written by mkfs.
Signed-off-by: David Scott <[email protected]>
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
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Signed-off-by: David Scott <[email protected]> Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
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Signed-off-by: Derek McGowan <[email protected]>
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Signed-off-by: Derek McGowan <[email protected]>
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@djs55 is this good to go now? Seems the testing of it has been successful |
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(slightly experimental, still being tested)
On Windows (NTFS/ReFS), extending a file with Truncate does not create a sparse file by default. When mkfs.ext4 subsequently writes metadata at scattered offsets across the file, NTFS must zero-fill all intervening blocks up to the valid data length before each write. For large filesystem images this is extremely slow.
Fix this by marking the file as sparse via FSCTL_SET_SPARSE before calling Truncate. This allows NTFS to leave unwritten regions unallocated, matching the behavior of macOS/Linux where Truncate already produces sparse files.
Benchmark results (8 GB file, Windows 11, NTFS):
~300x speedup for the file creation + write pattern, and on-disk usage drops from 8 GB to only the blocks actually written by mkfs.