Re: number of hardlinks for directory in ls -lid always 1?

[Filipe David Manana <fdmanana@gmail.com>]

On Thu, Mar 19, 2015 at 1:21 PM, David Sterba <dsterba@suse.cz> wrote:
> On Wed, Mar 18, 2015 at 03:23:50PM +0100, Martin Steigerwald wrote:
>> It explains that having a correct hardlink number for directory is not
>> mandatory, but it doesn´t explain why BTRFS always has 1 in there instead
>> of the actual count of hardlinks. Is this an performance optimization for
>> BTRFS or are there any other reasons why BTRFS does it this way?
>
> I believe it's for performance reasons. New inodes do not update the
> parent directory metadata wrt link counts, compared to other filesystems
> that do that.

Weird. Because creating a new inode implies adding the dentry to the
parent directory, which implies updating the directory's i_size.

>
> The real performance hit could be noticeable. The directory inode is
> cached in memory, so first update would be a bit slower, but the
> metadata block needs to be cow-ed on each new file. It's stress on
> b-tree locking and allocating new buffers for the metadata blocks.
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-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."