Re: File System Performance

Mike Fedyk
2001-11-13T01:29:02Z

On Mon, Nov 12, 2001 at 05:26:31PM -0700, Richard Gooch wrote:
> Mike Fedyk writes:
> > On Mon, Nov 12, 2001 at 05:04:57PM -0700, Richard Gooch wrote:
> > > 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"...
>

This is talking about the previous post only... The two messages above are
comment and reply, and are meant for each other. The part about tar moved
below...

> I assume you mean my scheme for tar. Which is an adaptation for
> user-space of a scheme that's been proposed for in-kernel ext2fs.
>

Nope, not one bit...

> > It deals very well with fast growth, but not so well with slow
> > growth, as mentioned in previous posts in this thread...
>
> Yes, yes. I know that.
>

Ok... We're getting somewhere.

> > There is a lengthy thread in ext2-devel right now, if you read it
> > it'll answer many of your questions.
>
> Is this different from the long thread that's been on linux-kernel?
>

Yes. The thread on ext2-devel will give you *much* more detail on the patch
you are trying to understand.

> Erm, I'm not really asking a bunch of questions. The only question I
> asked was whether I mis-read the current code, and that in turn is a
> response to your assertion that my scheme would not help, as part of
> an explanation of why it should work. Which you haven't responded to.

Hmm, I don't see the part I didn't reply to...

> If you claim my tar scheme wouldn't help, then you're also saying that
> the new algorithm for ext2fs won't help. Is that what you meant to
> say?
>

No, I'm not saying that...

> In any case, my point (I think you missed it, although I guess I
> didn't make it explicit) was that, rather than tuning the in-kernel
> algorithm for this fast-growth scenario, we may be better off adding
> an option to tar so we can make the choice in user-space. From the
> posts that I've seen, it's not clear that we have an obvious choice
> for a scheme that works well for both slow and fast growth cases.
>

Maybe not obvious, but with a little work, both can probably be made *better*.

> Having an option for tar would allow the user to make the choice.
> Ultimately, the user knows best (or at least can, if they care enough
> to sit down and think about it) what the access patterns will be.
>

Fixing tar won't help anyone except for tar users. What about the other
programs that create activity much like tar? This isn't a user space issue.

> However, I see that people are banging away at figuring out a generic
> in-kernel mechanism that will work with both slow and fast growth
> cases. We may see something good come out of that.
>

Yep

===================

Let me disect our previous conversation...

> > > > 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?
> > > > >

Currently, without any patching, any new directory will be put in a
different block group from its parent.

So, if you create the dirs in the same dir and then shuffle them around, you
gain nothing.

> > > > > 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.
> > > >

This doesn't fix all fast growth type apps, only tar and cp...

> > > Mike Fedyk writes:
> > > > 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...
> > >

Now does this make sence?

> > On Mon, Nov 12, 2001 at 05:04:57PM -0700, Richard Gooch wrote:
> > > 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.
> > >

Not currently, but the patch that is out will do this.

> > > 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.
> > >

If the kernel is patched...

> > > Am I wrong in my understanding of the current algorithm?
> >

Yes.

> Mike Fedyk writes:
> > You are almost describing the new algo to a "T"...
>

The above is a little more verbose, does it help?

Now, if I am not stating fact, as things currently are, and the state of
available patches are not what I am describing, someone please let me know.
Though to my understanding there is no error.

Mike