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As described in buffer-format.rst, the existing initramfs.c extraction
logic works fine if the cpio filename field is padded out with trailing
zeros, with a caveat that the padded namesize can't exceed PATH_MAX.
Add filename zero-padding logic to gen_init_cpio, which can be triggered
via the new -a <data_align> parameter. Performance and storage
utilization is improved for Btrfs and XFS workloads, as copy_file_range
can reflink the entire source file into a filesystem block-size aligned
destination offset within the cpio archive.
Btrfs benchmarks run on 6.15.8-1-default (Tumbleweed) x86_64 host:
> truncate --size=2G /tmp/backing.img
> /sbin/mkfs.btrfs /tmp/backing.img
...
Sector size: 4096 (CPU page size: 4096)
...
> sudo mount /tmp/backing.img mnt
> sudo chown $USER mnt
> cd mnt
mnt> dd if=/dev/urandom of=foo bs=1M count=20 && cat foo >/dev/null
...
mnt> echo "file /foo foo 0755 0 0" > list
mnt> perf stat -r 10 gen_init_cpio -o unaligned_btrfs list
...
0.023496 +- 0.000472 seconds time elapsed ( +- 2.01% )
mnt> perf stat -r 10 gen_init_cpio -o aligned_btrfs -a 4096 list
...
0.0010010 +- 0.0000565 seconds time elapsed ( +- 5.65% )
mnt> /sbin/xfs_io -c "fiemap -v" unaligned_btrfs
unaligned_btrfs:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..40967]: 695040..736007 40968 0x1
mnt> /sbin/xfs_io -c "fiemap -v" aligned_btrfs
aligned_btrfs:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..7]: 26768..26775 8 0x0
1: [8..40967]: 269056..310015 40960 0x2000
2: [40968..40975]: 26776..26783 8 0x1
mnt> /sbin/btrfs fi du unaligned_btrfs aligned_btrfs
Total Exclusive Set shared Filename
20.00MiB 20.00MiB 0.00B unaligned_btrfs
20.01MiB 8.00KiB 20.00MiB aligned_btrfs
XFS benchmarks run on same host:
> sudo umount mnt && rm /tmp/backing.img
> truncate --size=2G /tmp/backing.img
> /sbin/mkfs.xfs /tmp/backing.img
...
= reflink=1 ...
data = bsize=4096 blocks=524288, imaxpct=25
...
> sudo mount /tmp/backing.img mnt
> sudo chown $USER mnt
> cd mnt
mnt> dd if=/dev/urandom of=foo bs=1M count=20 && cat foo >/dev/null
...
mnt> echo "file /foo foo 0755 0 0" > list
mnt> perf stat -r 10 gen_init_cpio -o unaligned_xfs list
...
0.011069 +- 0.000469 seconds time elapsed ( +- 4.24% )
mnt> perf stat -r 10 gen_init_cpio -o aligned_xfs -a 4096 list
...
0.001273 +- 0.000288 seconds time elapsed ( +- 22.60% )
mnt> /sbin/xfs_io -c "fiemap -v" unaligned_xfs
unaligned_xfs:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..40967]: 106176..147143 40968 0x0
1: [40968..65023]: 147144..171199 24056 0x801
mnt> /sbin/xfs_io -c "fiemap -v" aligned_xfs
aligned_xfs:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..7]: 120..127 8 0x0
1: [8..40967]: 192..41151 40960 0x2000
2: [40968..40975]: 236728..236735 8 0x0
3: [40976..106495]: 236736..302255 65520 0x801
The alignment is best-effort; a stderr message is printed if alignment
can't be achieved due to PATH_MAX overrun, with fallback to non-padded
filename. This allows it to still be useful for opportunistic alignment,
e.g. on aarch64 Btrfs with 64K block-size. Alignment failure messages
provide an indicator that reordering of the cpio-manifest may be
beneficial.
Archive read performance for reflinked initramfs images may suffer due
to the effects of fragmentation, particularly on spinning disks. To
mitigate excessive fragmentation, files with lengths less than
data_align aren't padded.
Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-8-ddiss@suse.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.