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Add copy-on-write support to scm_copy_file.

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On modern file-systems (BTRFS, ZFS) it is possible to copy a file using
copy-on-write method.  For large files it has the advantage of being
much faster and saving disk space (since identical extents are not
duplicated).  This feature is stable and for example coreutils' `cp'
does use it automatically (see --reflink).

This commit adds support for this feature into our copy-file procedure.
Same as `cp', it defaults to 'auto, meaning the copy-on-write is
attempted, and in case of failure the regular copy is performed.

No tests are provided, because the behavior depends on the system,
underlying file-system and its configuration.  That makes it challenging
to write a test for it.  Manual testing was performed instead:

    $ btrfs filesystem du /tmp/cow*
         Total   Exclusive  Set shared  Filename
      36.00KiB    36.00KiB       0.00B  /tmp/cow

    $ cat cow-test.scm
    (copy-file "/tmp/cow" "/tmp/cow-unspecified")
    (copy-file "/tmp/cow" "/tmp/cow-always" #:copy-on-write 'always)
    (copy-file "/tmp/cow" "/tmp/cow-auto" #:copy-on-write 'auto)
    (copy-file "/tmp/cow" "/tmp/cow-never" #:copy-on-write 'never)
    (copy-file "/tmp/cow" "/dev/shm/cow-unspecified")
    (copy-file "/tmp/cow" "/dev/shm/cow-auto" #:copy-on-write 'auto)
    (copy-file "/tmp/cow" "/dev/shm/cow-never" #:copy-on-write 'never)
    $ ./meta/guile -s cow-test.scm

    $ btrfs filesystem du /tmp/cow*
         Total   Exclusive  Set shared  Filename
      36.00KiB       0.00B    36.00KiB  /tmp/cow
      36.00KiB       0.00B    36.00KiB  /tmp/cow-always
      36.00KiB       0.00B    36.00KiB  /tmp/cow-auto
      36.00KiB    36.00KiB       0.00B  /tmp/cow-never
      36.00KiB       0.00B    36.00KiB  /tmp/cow-unspecified

    $ sha1sum /tmp/cow* /dev/shm/cow*
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-always
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-auto
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-never
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-unspecified
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /dev/shm/cow-auto
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /dev/shm/cow-never
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /dev/shm/cow-unspecified

This commit also adds to new failure modes for (copy-file).

Failure to copy-on-write when 'always was passed in:

    scheme@(guile-user)> (copy-file "/tmp/cow" "/dev/shm/cow" #:copy-on-write 'always)
    ice-9/boot-9.scm:1676:22: In procedure raise-exception:
    In procedure copy-file: copy-on-write failed: Invalid cross-device link

Passing in invalid value for the #:copy-on-write keyword argument:

    scheme@(guile-user)> (copy-file "/tmp/cow" "/dev/shm/cow" #:copy-on-write 'nevr)
    ice-9/boot-9.scm:1676:22: In procedure raise-exception:
    In procedure copy-file: invalid value for #:copy-on-write: nevr

* NEWS: Add note for copy-file supporting copy-on-write.
* configure.ac: Check for linux/fs.h.
* doc/ref/posix.texi (File System)[copy-file]: Document the new
signature.
* libguile/filesys.c (clone_file): New function cloning a file using
FICLONE, if supported.
(k_copy_on_write): New keyword.
(sym_always, sym_auto, sym_never): New symbols.
(scm_copy_file2): Renamed from scm_copy_file.  New #:copy-on-write
keyword argument.  Attempt copy-on-write copy by default.
(scm_copy_file): Call scm_copy_file2.
* libguile/filesys.h: Add scm_copy_file2 as SCM_INTERNAL.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
This commit is contained in:
Tomas Volf 2024-01-24 20:14:32 +01:00 • committed by Ludovic Courtès
parent 9e0f03c5fd
commit e1690f3fd2
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6 changed files with 92 additions and 14 deletions
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@ -21,6 +21,15 @@ definitely unused---this is notably the case for modules that are only
used at macro-expansion time, such as (srfi srfi-26). In those cases,
the compiler reports it as "possibly unused".
** copy-file now supports copy-on-write
The copy-file procedure now takes an additional keyword argument,
#:copy-on-write, specifying whether copy-on-write should be done, if the
underlying file-system supports it. Possible values are 'always, 'auto
and 'never, with 'auto being the default.
This speeds up copying large files a lot while saving the disk space.
* Bug fixes
** (ice-9 suspendable-ports) incorrect UTF-8 decoding