Re: [E2FSPROGS, RFC] mke2fs: New bitmap and inode table allocation for FLEX_BG

[Theodore Tso <tytso@mit.edu>]

On Tue, Apr 22, 2008 at 09:18:47AM -0500, Jose R. Santos wrote:
> > @@ -1638,6 +1654,19 @@ static void PRS(int argc, char *argv[])
> > 
> >  	if (inode_size == 0)
> >  		inode_size = get_int_from_profile(fs_types, "inode_size", 0);
> > +	if (!flex_bg_size && (fs_param.s_feature_incompat &
> > +			      EXT4_FEATURE_INCOMPAT_FLEX_BG))
> > +		get_int_from_profile(fs_types, "flex_bg_size", 8);
> 
> A default of 256 block groups to pack seems a bit high base on some of
> the performance testing that I've done.  At some point having the inodes
> too far away from the data blocks begins to affect performance
> (especially on read operations).  The optimum number of groups depends
> a lot on platter density of the hard drive so I expect that we can
> increase the default grouping size as time goes by.  Using 128 groups
> as already showing performance degradation on read operations on some
> of my smaller disks (147GB).  For now, I would change this to 6 (64
> groups) as this is a good balance for both big an small disks.

Actually this is 8 (as in 2**3), which was intentionally very small,
because I was being conservative.  I could change it to be 64 if you
think it is a better balance.  As you can see, it gets set later on
down here.

> > +		fs_param.s_log_groups_per_flex = int_log2(flex_bg_size);

And, in fact the biggest bug which both you and I missed was that this:

> > +		get_int_from_profile(fs_types, "flex_bg_size", 8);

Should have been this:

       	    flex_bg_size = get_int_from_profile(fs_types, "flex_bg_size", 8);

<Dons paper bag>

						- Ted