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Date: Thu, 16 Mar 2006 08:53:41 -0500
From: "Theodore Ts'o" <tytso@mit.edu>
To: Takashi Sato <sho@bsd.tnes.nec.co.jp>
Cc: cmm@us.ibm.com, linux-kernel@vger.kernel.org,
       ext2-devel@lists.sourceforge.net
Subject: Re: [Ext2-devel] [PATCH 1/2] ext2/3: Support 2^32-1 blocks(Kernel)
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On Thu, Mar 16, 2006 at 09:11:17PM +0900, Takashi Sato wrote:
> >You changed most of the affected variables from "int" to "unsigned int",
> >that seems allow block number to address 2^32. It probably a good thing
> >to consider change the variables to sector_t type, so when the time we
> >want to support for 64 bit block number, we don't have to re-do the
> >similar work again.  Laurent did very similar work on this before.
> 
> sector_t is 8bytes on normal configuration and there are many
> variables for blocks on ext2/3.  I thought extending variables may
> influence on performance, so I didn't change.

It would be interesting to do a CPU overhead benchmark to see how much
of the overhead is actually measurable on an x86 system.  If it's only
a small percent, it might be acceptable given that x86_64 machines are
going to be gradually taking over, and sector_t only exists if
CONFIG_LBD is enabled.  So for smaller systems where LBD isn't
enabled, we won't see performance overhead since sector_t won't exist
and so the code is going to have to use a typedef for ext2_blk_t which
is either __u32 or sector_t as necessary.

> >- The superblock format currently stores the number of block groups as a
> >16-bit integer, and because (on a 4 KB blocksize filesystem) the maximum
> >number of blocks in a block group is 32,768 , a combination of these
> >constraints limits the maximum size of the filesystem to 8 TB
> 
> Is it s_block_group_nr in ext3_super_block?
> mke2fs sets 65535 to the field if the number of block groups is greater
> than 65535.  Current kernel ignores the field and re-calculate from
> other fields.  findsuper command is the only user of it and it simply prints
> the value.  So, it does not limit the maximum size of the filesystem to 8 
> TB.

s_block_group_nr is *not* the number of block groups in the
filesystem.  As Takashi-san properly pointed out, the kernel
calculates the number of block groups by dividing the number of blocks
by the blocks_per_group fields.  s_block_group_nr is used to identify
the block group of a particular backup supeblock.  

So for the backup superblock located at block group #3,
s_block_group_nr 3, and for the backup superblock located at block
group #5, s_block_group_nr 5, and so on.  It is used only as a hint so
that prorams like findsuper and gpart can be more intelligent about
finding the start of filesystem, when trying to recover from a smashed
partition table.

						- Ted
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