Date:	Sun, 17 May 2009 19:49:09 -0700 (PDT)
From:	david@...g.hm
To:	Theodore Tso <tytso@....edu>
cc:	Timo Sirainen <tss@....fi>, Josef Bacik <josef@...icpanda.com>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: ext3/ext4 directories don't shrink after deleting lots of
 files

On Sun, 17 May 2009, Theodore Tso wrote:

> On Thu, May 14, 2009 at 08:45:38PM -0400, Timo Sirainen wrote:
>> I was rather thinking something that I could run while the system was
>> fully operational. Otherwise just moving the files to a temp directory +
>> rmdir() + rename() would have been fine too.
>>
>> I just tested that xfs, jfs and reiserfs all shrink the directories
>> immediately. Is it more difficult to implement for ext* or has no one
>> else found this to be a problem?
>
> I've sketched out a design that shouldn't be too hard to implement
> that will address the problem which you've raised.  I'm not sure when
> I will have to implement it, so in case there's an ext4 developer who
> has time, I thought I would throw it out there.  For folks who are
> looking for something simple to get started, perhaps after submitting
> a few bug fixes or cleanups, this should be a fairly straight forward
> project.
>
> The constraints that we have is that for backwards compatibility's
> sake, we can't support spares directories.  So if a block in the of

s/spares/sparse/ ?

> the directory becomes empty, we can't just unallocate it unless the it
> is at the very end of the directory.  In addition, if htree support is
> enabled, we also need to make sure the hash tree index is updated
> remove the reference to the block we are about to remove.  Finally, if
> journalling is enabled, we need to know in advance how many blocks the
> unlink() operations will need to touch.
>
> So the basic design is as follows.  We add a new parameter to
> ext4_delete_entry(), which is a pointer to a new data structure,
> ext4_dir_cleanup.  This it gets filled in with information about the
> directory block containing the directory entry which was removed:
> directory inode, logical and physical block number, the directory
> index blocks if present, etc.  Then the callers of ext4_delete_entry()
> (ext4_rmdir, ext4_rename, and ext4_unlink) take that information ad
> pass it another function which takes tries to shrink the directory ---
> but this function gets called *after* the call to ext4_journal_stop().
> That way we don't have to change any of the journal accounting credits
> and the ext4_shrink_directory() function is does purely optional work.
>
> At least initially, the ext4_shrink_directory() might only do
> something useful if the last directory block in the directory is
> empty, and htree is not enabled; in that case, it can just simply
> truncate the last block, and return.
>
> The next step would be to teach ext4_shrink_directory() how to handle
> removing the last directory block for htree directories; this means
> that it will need to find the the entry in the htree index block, and
> remove the entry in the htree index.
>
> Next, to handle the case where the empty directory block is *not* the
> last block in the directory, what ext4_shrink_directory() can do is to
> take the contents of the last directory block, and copy it to the
> empty directory block, and then do the truncate operation.  In the
> case of htree directories, the htree index blocks would also have to
> be updated (both removing the index entry pointing to the empty
> directory block, as well as updating the index entry which had been
> pointing to the last directory block).

I think this is more complex. I think you can't just move the last 
directory block to one earlier because that would change the order of 
things in the directory, messing up things that do a partial readdir of 
the directory and then come back to pick up where they left off. you would 
need to move all blocks after the empty up one.

Another thing, you don't nessasarily want to do this movement immediatly 
when a directory block becomes empty. It's very possible that the user is 
deleting a lot of things from the directory, and so may delete enough 
stuff so that all (or almost all) of the directory blocks could be deleted 
through the 'last block in the directory' method. it may be that the best 
thing to do at this point is to wait for instructions from the user to do 
this (more below)

> Finally, ext4_shrink_directory() could be taought how to take an
> *almost* empty directory block, and attempts to move the directory
> entries to the previous and/or next directory block.

this sounds like something that's best implemented as a nighly cron job 
(run similar to updatedb) to defrag the directory blocks. given changes 
over the years to disk technology (both how much slower seeks have become 
relative to sequential reads on rotating media, and how SSDs really have 
much larger block sizes internally than what's exposed to users), it may 
make sense to consider having a defrag tool for the data blocks as well.

David Lang

> The basic idea is that ext4_shrink_directory() could be implemented
> and tested incrementally, with at each stage it becoming more
> aggressive about being able to shrink directories.
>
> Anyway, if there's someone interested in trying to implement this,
> give me a holler; I'd be happy to give more details as necessary.
>
>     	  	      	       	  	- Ted
>
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