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

[Theodore Tso <tytso@mit.edu>]

On Tue, Apr 22, 2008 at 10:32:12AM -0500, Jose R. Santos wrote:
> 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.

Let's stay with 16 then for now.  Spindle speed doesn't actually
matter here; what matters is seek speed, and the density of the disk
drive.  The other thing which worries me though is that the size of
each flex_bg block group cluster is dependent on the size of the block
group, which in turn is related to the square of the filesystem
blocksize.   i.e., assuming a fs blockgroup size of 16, then:

Blocksize    Blocks/blockgroup  Blockgroup Size   Flex_BG cluster size

   1k	         8192             8 Meg	              128 Meg
   2k           16384             32 Meg              512 Meg
   4k           32768		  128 Meg	      2 Gig
   8k		65536             512 Meg	      8 Gig
  16k          131072             2 Gig		      32 Gig
  32k	       262144		  8 Gig		      128 Gig
  64k	       524288		  32 Gig	      512 Gig

So using a fixed default of 16, the flexible blockgroup size can range
anything from 128 megs to half a terabyte!

How much a difference in your numbers are you seeing, anyway?  Is it
big enough that we really need to worry about it?

    	   	   	       	  	- Ted