Re: File System Performance

[Mike Fedyk]

On Mon, Nov 12, 2001 at 05:04:57PM -0700, Richard Gooch wrote:
> Mike Fedyk writes:
> > On Mon, Nov 12, 2001 at 12:59:54PM -0700, Richard Gooch wrote:
> > > Here's an idea: add a "--compact" option to tar, so that it creates
> > > *all* inodes (files and directories alike) in the base directory, and
> > > then renames newly created entries to shuffle them into their correct
> > > positions. That should limit the number of block groups that are used,
> > > right?
> > > 
> > > It would probably also be a good idea to do that for cp as well, so
> > > that when I do a "cp -al" of a virgin kernel tree, I can keep all the
> > > directory inodes together. It will make a cold diff even faster.
> > 
> > I don't think that would help at all... With the current file/dir
> > allocator it will choose a new block group for each directory no
> > matter what the parent is...
> 
> I thought the current implementation was that when creating a
> directory, ext2fs searches forward from the block group the parent
> directory is in, looking for a "relatively free" block group. So, a
> number of successive calls to mkdir(2) with the same parent directory
> will result in the child directories being in the same block group.
> 
> So, creating the directory tree by creating directories in the base
> directory and then shuffling should result in the directories be
> spread out over a modest number of block groups, rather than a large
> number.
> 
> Addendum to my scheme: leaf nodes should be created in their
> directories, not in the base directory. IOW, it's only directories
> that should use this trick.
> 
> Am I wrong in my understanding of the current algorithm?
> 

You are almost describing the new algo to a "T"...

It deals very well with fast growth, but not so well with slow growth, as
mentioned in previous posts in this thread...

There is a lengthy thread in ext2-devel right now, if you read it it'll
answer many of your questions.

Mike