linux-ext4 - Re: Max blocks per group?

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Message-ID: <20120628192303.GA9054@thunk.org>
Date:	Thu, 28 Jun 2012 15:23:03 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Phillip Susi <psusi@...ntu.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Max blocks per group?

On Thu, Jun 28, 2012 at 11:16:31AM -0400, Phillip Susi wrote:
> 
> I notice that mke2fs refuses to allow more than 8 * block size blocks
> per group.  Why is this?  Is it not permissible to have more than one
> block for the block allocation bitmap?

No, it's not permissible; it's been a fundamental design constraint in
ext2/3/4 since the very beginning.  You can, however, get the moral
equivalent by using flex_bg which moves the metadata blocks to the
beginning of an aggregated of (for example) 16 block groups of a
flex_bg.  The block allocation bitmaps are allocated contiguously, so
between that and locating all of the metadata blocks of the bg's at
the beginning of a flex_bg, that should get most of what you'd want
with a larger number of blocks in a block group.

And BTW, this is already the default in ext4 (using 16 block groups in
each flex_bg).

						- Ted
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