On Tue, Jan 29, 2013 at 11:41:13AM +0400, Dmitry Monakhov wrote:
> On Mon, 28 Jan 2013 10:38:36 -0500, Theodore Ts'o <tytso@xxxxxxx> wrote:
> > On Mon, Jan 28, 2013 at 07:02:55PM +0400, Dmitry Monakhov wrote:
> > > Actually this patch consists of two peaces
> > > 1) disable merging of uninitialized extents. (1 line change) I'm
> > > absolutely agree with it.
> >
> > To be clear, that's this patch chunk (one line change not including
> > comments :-), right?
> Off course.
> >
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -1579,11 +1576,13 @@ int
> > ext4_can_extents_be_merged(struct inode *inode, struct ext4_extent *ex1,
> > struct ext4_extent *ex2)
> > {
> >
> > /*
> > - * Make sure that either both extents are uninitialized, or
> > - * both are _not_.
> > + * Make sure that both extents are initialized. We don't merge
> > + * uninitialized extents so that we can be sure that end_io code has
> > + * the extent that was written properly split out and conversion to
> > + * initialized is trivial.
> > */
> > - if (ext4_ext_is_uninitialized(ex1) ^ ext4_ext_is_uninitialized(ex2))
> > + if (ext4_ext_is_uninitialized(ex1) || ext4_ext_is_uninitialized(ex2))
> > return 0;
> >
> >
> > The one thing I'm a bit worried about is how much worse will extent
> > fragmentation be once we do this, but it's clear we need to strive for
> > correctness first.
> This change should not affect fragmentation because
> 1) Most people call fallocate(2) on big chunks (>4M)
> 2) Once uninitialized extent filled with data and converted to
> initialized extents will be merged immediately.
> 3) Most people use fallocate(2) for preallocation before write(2)
> so effectively calls are interleaved so merging works as expected.
>
> The only case where fragmentation will increase is when someone
> performs many fallocate(2) calls for small chunks (4k) w/o writes.
> As result leaf block will consist of 256 extents 4k each.
> Later writes can't help us because we can not merge extents from two
> leaf blocks. But I still think that this use case it inconvenient.
Yes, I doubt that no one does like this because it can not brings any
benefit. We usually call fallocate(2) to preallocate some sequential
spaces. Obviously preallocating a small chunk is useless.
>
> BTW why do we not try to merge extents from two leaf blocs?
> I do not see any technical difficulties. If two adjacent leaf blocks
> are covered by common index block merging is possible (but we need +1
> journal block).
Yep, I also notice this. I don't think there is a technical difficulty.
That would be great if two adjacent leaf blocks could be merged.
Regards,
- Zheng
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