Re: posix_fallocate behavior in glibc

[Florian Weimer <fweimer@redhat.com>]

* Christoph Hellwig:

> The glibc implementation in sysdeps/posix/posix_fallocate.c, which is
> also by sysdeps/unix/sysv/linux/posix_fallocate.c as a fallback if the
> fallocate syscall returns EOPNOTSUPP is implemented by doing single
> byte writes at intervals of min(f.f_bsize, 4096).

> How can we get rid of this glibc fallback that turns the implementations
> non-conformant and increases write amplication for no good reason?

When does the kernel return EOPNOTSUPP these days?  We do not even do
fallback for EPERM/ENOSYS, those that might be encountered in
containers.

Last time I looked at this I concluded that it does not make sense to
push this write loop from glibc to the applications.  That's what would
happen if we had a new version of posix_fallocate that didn't do those
writes.  We also updated the manual:

  Storage Allocation
  <https://sourceware.org/glibc/manual/latest/html_node/Storage-Allocation.html>

As mentioned, if an application doesn't want fallback behavior, it can
call fallocate directly.

Thanks,
Florian