> This is precisely the problem which needs to be solved for delayed
> allocation on ext2/3/4. This is because it is infeasible to work out how
> much disk space an ext2 pagecache page will take to write out (it will
> require zero to three indirect blocks as well).
>
> When I did delalloc-for-ext2, umm, six years ago I did
> maximally-pessimistic in-memory space accounting and I think I just ran a
> superblock-wide sync operation when ENOSPC was about to happen. That
> caused all the pessimistic reservations to be collapsed into real ones,
> releasing space. So as the disk neared a real ENOSPC, the syncs becaome
> more frequent. But the overhead was small.
>
> I expect that a similar thing was done in the ext4 delayed allocation
> patches - you should take a look at that and see what can be
> shared/generalised/etc.
>
>
ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/ext4-patches/LATEST/broken-out/
>
> Although, judging by the comment in here:
>
>
ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/ext4-patches/LATEST/broken-out/ext4...
>
> + * TODO:
> + * MUST:
> + * - flush dirty pages in -ENOSPC case in order to free reserved blocks
>
> things need a bit more work. Hopefully that's a dead comment.
>
> <looks>
>
> omigod, that thing has gone and done a clone-and-own on half the VFS.
> Anyway, I doubt if you'll be able to find a design description anyway
> but you should spend some time picking it apart. It is the same problem..
