Re: [PATCH 0/3] ext4: introduce two new ioctls

[Dave Chinner <david@fromorbit.com>]

On Sun, Jun 23, 2013 at 11:12:35PM -0400, Theodore Ts'o wrote:
> On Mon, Jun 24, 2013 at 12:44:59PM +1000, Dave Chinner wrote:
> > 
> > Hence, at minimum, this should be a fallocate() operation, not a ext4
> > specific ioctl as it is relatively trivial to implement on most
> > extent based filesystems.
> 
> The fallocate() uses a units of bytes for the offset and length; would
> a FALLOC_FL_COLLAPSE_RANGE be guaranteed to work on any arbitrary
> offset and length?  Or would it only work if the offset and length are
> multiples of the file system blocksize?

There's nothing stopping us from restricting the offset/len to
specific alignments if the operation cannot be done on arbitrary
byte ranges. We do that for direct IO, and the sky hasn't fallen
yet.

> The the EXT4_IOC_TRUNCATE_BLOCK_RANGE interface solves this problem by
> using units of file system blocks (i.e., __u32 start_block), but that
> raises another issue, which is it forces the user space program to
> somehow figure out the file system block size, which seems a bit nasty.

Yeah, exactly. We can do that internally very easily, and EINVAL can
be returned when the alignment is bad just like we do for direct
IO...

But, well, I pine for a generic XFS_IOC_DIOINFO interface so the
filesystem can tell users about alignment restrictions....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com