Re: [Ext2-devel] disk throughput

[Alexander Viro <viro@math.psu.edu>]

On Mon, 5 Nov 2001, Linus Torvalds wrote:

> 
> On Mon, 5 Nov 2001, Alexander Viro wrote:
> >
> > OK, some digging had brought another one:
> >
> > a) if it's first-level directory - get it the fsck out of root's cylinder
> > group.
> 
> Hey, now that you've read it in a paper you like it, but when I suggest it
> in email you shoot it down?
> 
> <Whiny mode on>  I thought you loved me, Al.  <Whiny mode off>

Oh, come on. (a) is obvious, but obviously not enough ;-)
 
> > b) if we just keep creating directories in a cylinder group and do not
> > create any files there - stop, it's no good (i.e. there's a limit on
> > number of back-to-back directory creations in the same group).
> 
> The current code actually has some vestiges that _seem_ to be trying to do
> something like this: see the commented-out
> 
> 	if (tmp && le16_to_cpu(tmp->bg_used_dirs_count) << 8) <
> 		   le16_to_cpu(tmp->bg_free_inodes_count)) {
> 
> which _seems_ to want to play games with "number of directories allocated
> vs nr of free inodes".
> 
> But it's commented out with "I am not yet convinced that this next bit is
> necessary". I don't know if the code has ever been active, or whether it
> showed other problems.
>
> > c) try putting it into the parent's CG, but reserve some number of inodes
> > and data blocks in it.  If we can't - tough, get the fsck out of there.
> 
> Hmm.. Maybe this is actually closer to what we try to do above..

Yes, but block reservation also makes sense (otherwise we can end up
putting a directory into parent's CG only to have all children
going there _and_ getting far from their data).  Which might be the
problem with original code, BTW.

OK, anyway - I've got a bunch of cleanups for ialloc.c (equivalent
transformations, split into small steps and decently tested - I've
used them for almost a year).  That stuff moves choice of CG for
directories and non-directories into separate functions, so no
matter which variant we end up doing I think that it's worth doing
first - things will be cleaner after that.