Re: [PATCH 4/5] ext4: fallocate support in ext4

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Mon, 7 May 2007 15:21:04 -0700
Andreas Dilger <adilger@xxxxxxxxxxxxx> wrote:

> On May 07, 2007  13:58 -0700, Andrew Morton wrote:
> > Final point: it's fairly disappointing that the present implementation is
> > ext4-only, and extent-only.  I do think we should be aiming at an ext4
> > bitmap-based implementation and an ext3 implementation.
> 
> Actually, this is a non-issue.  The reason that it is handled for extent-only
> is that this is the only way to allocate space in the filesystem without
> doing the explicit zeroing.  For other filesystems (including ext3 and
> ext4 with block-mapped files) the filesystem should return an error (e.g.
> -EOPNOTSUPP) and glibc will do manual zero-filling of the file in userspace.

hrm, spose so.

It can be a bit suboptimal from the layout POV.  The reservations code will
largely save us here, but kernel support might make it a bit better.

Totally blowing pagecache could be a problem.  Fixable in userspace by
using sync_file_range()+fadvise() or O_DIRECT, but I bet it doesn't.