On 3/14/11 3:52 PM, Phillip Susi wrote:
> 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.
readdir uses htree / dir_index:
ext3_readdir()
if (EXT3_HAS_COMPAT_FEATURE(inode->i_sb,
EXT3_FEATURE_COMPAT_DIR_INDEX) &&
((EXT3_I(inode)->i_flags & EXT3_INDEX_FL) ||
((inode->i_size >> sb->s_blocksize_bits) == 1))) {
err = ext3_dx_readdir(filp, dirent, filldir);
Because dir_index places entries into blocks in hash order, reading
it "like a non-dir_index" dir still gives you out of order entries,
I think. IOW it doesn't slow down readdir, it just gives you a nasty
order - slowing down access to those files.
> 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.
Nope, it's used for all directories AFAIK. Certainly shows the most
improvement on lookups in large directories though...
> 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.
Yeah, this has been a longstanding nastiness...
-Eric
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