From: "Jose R. Santos" <jrs@us.ibm.com>
Subject: Re: [E2FSPROGS, RFC] mke2fs: New bitmap and inode table allocation
 for FLEX_BG
Date: Tue, 22 Apr 2008 10:32:12 -0500
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Cc: linux-ext4@vger.kernel.org,
	Valerie Clement <valerie.clement@bull.net>
To: Theodore Tso <tytso@mit.edu>
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On Tue, 22 Apr 2008 10:51:25 -0400
Theodore Tso <tytso@mit.edu> wrote:

> 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.

I see that now, guess I should not read code with out having
breakfast.  I think 8 is a very safe and conservative number, maybe to
conservative. The 64 group packing was the number I found to be a
overall improvement with the limited number of drives that I had to
test with.  Haven't done any testing on old drives or laptop drive with
slow spindle speed but I would think 16 or 32 would be safe here unless
the drive is really old and small.

> 
> > > +		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



-JRS