bug#9500: [PATCH]: use posix_fallocate where supported

[Jim Meyering]

Pádraig Brady wrote:
...
> I thought a little about this today.

Nice description of the issues.

It's probably worth putting something like this somewhere in version
control, even if only as a long commit message on whatever change you make.

> fallocate() is a feature to quickly allocate space in a file system.
> It's useful for 3 things as far as I can see:
>
>   1. Improved file layout for subsequent access
>   2. Immediate indication of ENOSPC
>   3. Efficient writing of NUL portions
>
> Note 1. is somewhat moot with newer file systems that do "delayed allocation".
> So what do we need to consider when using fallocate on the destination file?
> Considering just cp for the moment, its inputs impacting this are the options:
>
...
> Copying sparse files
>
>  It's worth noting again, the caveat mentioned above that we
>  might not recognise some sparse files due to tail allocation.

Yes, this is worth repeating ;-)
It is surprising, at least in part because significant tail
allocation is not common.

>  Given that we use fiemap (with sync) for sparse files at present,
>  we can augment the fiemap copying code to use fallocate where appropriate.
>   So dependent on the options the operations would be:
>     --sparse=auto   => 'Empty' -> 'Empty'
>     --sparse=always => 'Empty' -> 'Hole'  && discard tail allocation
>     --sparse=never  => 'Hole'  -> 'Empty'
>  Perhaps the first case could be simplified to initially doing:
>     fallocate(dest, blocks*blocksize))
>
> Copying normal files
>
>  Note using SEEK_HOLE for this case, would only help
>  to avoid reading 'Hole' and more likely 'Empty' portions,
>  and should not impact on the use of fallocate(dest).
>
>  So assuming we initially did:
>
>    if ! --sparse=always
>      fallocate(dest, st_size)
>
>  That would throw away any tail allocation in the source,
>  which is probably OK as noted above. In fact we might always
>  discard tail allocation for consistency, unless we can use fiemap
>  for all cases.

All sounds reasonable.

> I'll cook something up on this soon.

Thanks.