bug#79267: cp --sparse=auto heuristic fails on a squashfs mounted drive.

On 2025-08-23 12:03, Paul Eggert wrote:
> On 2025-08-23 11:46, Phillip Lougher wrote:
>> As far as Squashfs is concerned SEEK_HOLE/SEEK_DATA is easy to 
>> implement.  So I'll think about adding it as a build option.
> 
> Thanks, that'll be helpful.
> 
>> But this isn't going to fix it for any other case.
> 
> Right, and bleeding-edge coreutils already has a (slowish) workaround 
> for Squashfs as-is, as well as for other file systems that don't expose 
> extents to user code. If I get around to it I will install similar 
> workarounds in other user code I help maintain.

Phillip, as it turns out "slowish" was an understatement, as the 
performance regression was so bad for OpenZFS that we plan to stop using 
the workaround in the next release of GNU Coreutils. This change will 
resurrect coreutils bug #79267, i.e., coreutils copying from squashfs 
filesystems will no longer look for holes, and will create unnecessary 
explicit zeros in the output. For details, please see:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=79267#54
https://github.com/coreutils/coreutils/issues/122

Implementing SEEK_HOLE and SEEK_DATA for squashfs would be a better way 
to fix the squashfs+coreutils performance bug. It's OK if squashfs uses 
128 KiB or 1 MiB block sizes for this, as Coreutils will work fine 
either way.