On Wed, Aug 27, 2014 at 03:55:16PM +0200, Jan Kara wrote:
> On Thu 07-08-14 11:35:50, Zheng Liu wrote:
> > From: Zheng Liu <wenqing.lz@xxxxxxxxxx>
> >
> > Currently extent status tree doesn't cache extent hole when a write
> > looks up in extent tree to make sure whether a block has been allocated
> > or not. In this case, we don't put extent hole in extent cache because
> > later this extent might be removed and a new delayed extent might be
> > added back. But it will cause a defect when we do a lot of writes.
> > If we don't put extent hole in extent cache, the following writes also
> > need to access extent tree to look at whether or not a block has been
> > allocated. It brings a cache miss. This commit fixes this defect.
> > Meanwhile, if an inode has no any extent, this extent hole also will
> > be cached.
> >
> > Cc: "Theodore Ts'o" <tytso@xxxxxxx>
> > Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Signed-off-by: Zheng Liu <wenqing.lz@xxxxxxxxxx>
> So I agree with this change, it certainly doesn't make things worse. But
> when looking into ext4_ext_put_gap_in_cache() I have one question: That
> function uses ext4_find_delalloc_range() to check that the intended range
> doesn't have any delalloc blocks in it. However it doesn't make any effort
> to put a smaller hole in cache - for example if we have blocks allocated
> like:
>
> 5-6 = delalloc
> 7-10 = allocated
>
> and ext4_ext_put_gap_in_cache() is called for block 0, the function won't
> put anything into cache although it could put range 0-4 in the cache as a
> hole. Why is that?
Oops, it should put range 0-4 in the cache. Thanks for pointing it out.
I will fix it.
Regards,
- Zheng
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