Collapsing ranges in sparse files in APFS?

[Jens Alfke]

I’m looking into sparse-file operations in APFS. I see I can use fcntl to preallocate space and to punch holes in a file. But I don’t see a command to remove a byte range from a file without leaving a hole, equivalent to the
FALLOC_FL_COLLAPSE_RANGE mode of Linux’s fallocate, described thus:

> Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available since Linux 3.15) in mode removes a byte range from a file, without leaving a hole. The byte range to be collapsed starts at offset and continues for len bytes. At the completion of the operation, the contents of the file starting at the location offset+len will be appended at the location offset, and the file will be len bytes smaller.

Is there such an operation in Darwin, with an APFS filesystem?

—Jens
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[Chris Sarcone]

Hi Jens —

On Jan 20, 2019, at 3:14 PM, Jens Alfke <je...@mooseyard.com> wrote:

I’m looking into sparse-file operations in APFS. I see I can use fcntl to preallocate space and to punch holes in a file. But I don’t see a command to remove a byte range from a file without leaving a hole, equivalent to the
FALLOC_FL_COLLAPSE_RANGE mode of Linux’s fallocate, described thus:

Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available since Linux 3.15) in mode removes a byte range from a file, without leaving a hole. The byte range to be collapsed starts at offset and continues for len bytes. At the completion of the operation, the contents of the file starting at the location offset+len will be appended at the location offset, and the file will be len bytes smaller.

Is there such an operation in Darwin, with an APFS filesystem?

No, but you can file a Radar bug asking for the feature and we can evaluate it for inclusion.

Thanks,

— Chris

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[Jens Alfke]

On Jan 21, 2019, at 5:19 PM, Chris Sarcone <sar...@apple.com> wrote:

No, but you can file a Radar bug asking for the feature and we can evaluate it for inclusion.

OK, filed as rdar://47450388. 

(The Bug Reporter site didn’t have any category for “Filesystem”, “Core OS” or “Kernel”, so I chose “Something not listed here”. I’m smiling to myself at the irony that operating systems have grown to such a degree that something as fundamental as the filesystem no longer merits a place in a feature list!)

—Jens