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

[Kai Krakow <hurikhan77@gmail.com>]

Filipe David Manana <fdmanana@gmail.com> schrieb:

> 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.

Maybe related to snapshots? Which brings us back to COW and performance:

>> 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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> 
> 
> 
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