Sparse file handling

[jbd]

Hello,

Unless I'm mistaken, cpz does not handle sparse file and will copy null bytes over the wire. The xcp project (https://github.com/tarka/xcp) tries to iterate between sparse chunk and use copy_file_range for actual data.

I think this would be a nice addition and I hope you will consider this feature.

Sorry this is really an RFE without an associated pull request, but my rust skills are non-existent at the moment.

Thank you for this project. This is a fantastic contribution to the space of data movement.

Jean-Baptiste

[SUPERCILEX]

I looked into and it seems like a bit of a pain. You'd want to use this statx to determine if the file is sparse (by seeing if it's size on disk aka number of blocks is less than it's apparent size): https://github.com/SUPERCILEX/fuc/blob/2d61845b8f61f4332ef3b5a67842979bd578eeb4/fuc_engine/src/ops/copy.rs#L529

The to copy the file sparsely, I believe you can just use https://man7.org/linux/man-pages/man2/lseek.2.html alternating between seek_hole and seek_data to get the ranges that should be passed into copy_file_range. Then the end of the file might need an ftruncate to finish it off. Or maybe it's better to actually ftruncate the whole file at the beginning? Not sure.

[jbd]

Thank you for your answer !

I don't know about the ftruncate at the beginning. I think that https://github.com/hpc/mpifileutils is doing it in one of its pass (files and directories creation, data copy, setting permissions).

I agree that the sparse case looks quite tedious to implement:

https://github.com/tarka/xcp/blob/2253d6ffc1ca13e8de395700b7e4f3fd57fa42b5/libfs/src/linux.rs#L78 https://github.com/tarka/xcp/blob/2253d6ffc1ca13e8de395700b7e4f3fd57fa42b5/libxcp/src/operations.rs#L118 https://github.com/tarka/xcp/blob/2253d6ffc1ca13e8de395700b7e4f3fd57fa42b5/libxcp/src/operations.rs#L82 https://github.com/tarka/xcp/blob/2253d6ffc1ca13e8de395700b7e4f3fd57fa42b5/libfs/src/linux.rs#L206

Copying TB sparse files is not ideal too =)

In the end, this is a gentle request for enhancement, I just wanted to have your thoughts on this. I understand perfectly that you are more focused on interesting development like using io_uring.

Feel free to close this case !

Cheers.

Jean-Baptiste

[SUPERCILEX]

Copying TB sparse files is not ideal too =)

Lol, fair.

Feel free to close this case !

No no, I'm just saying that I probably won't implement this myself, but I'd be happy to accept a PR.