On Mon, Sep 01, 2014 at 10:43:50PM -0400, Theodore Ts'o wrote:
> On Thu, Aug 07, 2014 at 11:35:50AM +0800, 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.
>
> Hi Zheng,
>
> I thought the reason why we have the EXT4_GET_BLOCKS_NO_PUT_HOLE flag
> is because in ext4_da_map_blocks(), if there is a hole, we will be
> immediately following it up with a call to ext4_es_insert_extent() to
> fill in the hole with the EXTENT_STATUS_DELAYED flag. The only time
> we don't is if we run into an ENOSPC error.
>
> Am I missing something?
Yes, you are right. The purpose is used to do the work like you said
above. As the commit log described this flag brings a huge number of
cache misses when an empty file is written. So it might be worth
putting a hole in the cache when delalloc is enabled.
Regards,
- Zheng
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
