Re: Large directories and poor order correlation

[Phillip Susi <psusi@cfl.rr.com>]

On 3/14/2011 4:37 PM, Eric Sandeen wrote:
> On 3/14/11 3:24 PM, Phillip Susi wrote:
>> Shouldn't copying or extracting or otherwise populating a large
>> directory of many small files at the same time result in a strong
>> correlation between the order the names appear in the directory, and the
>> order their data blocks are stored on disk, and thus, read performance
>> should not be negatively impacted by fragmentation?
> 
> No, because htree (dir_index) dirs returns names in hash-value
> order, not inode number order.  i.e. "at random."

I thought that the htree was used to look up names, but the normal
directory was used to enumerate them?  In other words, the htree speeds
up opening a single file, but slows down traversing the entire
directory, so should not be used there.

Also isn't htree only enabled for large directories?  I still see crummy
correlation for small ( < 100 files, even one with only 8 entries )
directories.

It seems unreasonable to ask applications to read all directory entries,
then sort them by inode number to achieve reasonable performance.  This
seems like something the fs should be doing.