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From: Eric Sandeen <sandeen@redhat.com>
To: ext4 development <linux-ext4@vger.kernel.org>
Subject: Re: "- 8" in EXT2_MAX_BLOCKS_PER_GROUP
Date: Mon, 10 Oct 2011 13:43:28 -0500	[thread overview]
Message-ID: <4E933CD0.4050003@redhat.com> (raw)
In-Reply-To: <4E931453.3010002@redhat.com>

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

  reply	other threads:[~2011-10-10 18:43 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-10-10 15:50 "- 8" in EXT2_MAX_BLOCKS_PER_GROUP Eric Sandeen
2011-10-10 18:43 ` Eric Sandeen [this message]
2011-10-10 20:11   ` Darrick J. Wong
2011-10-10 20:34     ` Eric Sandeen

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