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

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Message-ID: <20111010201151.GM12447@tux1.beaverton.ibm.com>
Date:	Mon, 10 Oct 2011 13:11:51 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: "- 8" in EXT2_MAX_BLOCKS_PER_GROUP

On Mon, Oct 10, 2011 at 01:43:28PM -0500, Eric Sandeen wrote:
> 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?

As far as I know, each group contains a block bitmap that is exactly 1 block
long, and block can be no longer than 4096 bytes in length.  Therefore, a group
can have no more than 4096 * 8 = 32768 blocks, correct?  To get more we'd have
to allow blocks larger than 4K (mkfs won't allow that) or change the disk
format to allow multi-block block bitmaps.

Unless of course a block bitmap can be the size of a cluster.... which afaict
is not the case.

--D
> 
> -Eric
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