linux-ext4 - Re: "- 8" in EXT2_MAX_BLOCKS_PER

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Date:	Mon, 10 Oct 2011 13:43:28 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: "- 8" in EXT2_MAX_BLOCKS_PER_GROUP

On 10/10/11 10:50 AM, Eric Sandeen wrote:
> When looking at the maximal filesystem size issue, I found myself
> wondering what the "- 8" is in here, it's not commented
> or documented anywhere:
> 
> #define EXT2_MAX_BLOCKS_PER_GROUP(s)    (((1 << 16) - 8) *      \
>                                           (EXT2_CLUSTER_SIZE(s) / \
>                                            EXT2_BLOCK_SIZE(s)))
> 
> 
> (pre-bigalloc, it was just ((1 << 16) - 8) )
> 
> Anyone know?

Ah, Darrick pointed out

http://osdir.com/ml/file-systems.ext2.devel/2006-03/msg00032.html

So it would have been - 1, to not overflow __u16, but since we have
multiples of 8, we get - 8.

But now we have bg_free_blocks_count_hi, giving us 32 bits of counter.
With EXT4_FEATURE_INCOMPAT_64BIT, MAX_BLOCKS_PER_GROUP should grow, no?

-Eric
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