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From: Daniel Phillips <phillips@bonn-fries.net>
To: Andreas Dilger <adilger@clusterfs.com>
Subject: Re: [PATCH] remove 2TB block device limit
Date: Fri, 17 May 2002 03:17:19 +0200
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On Friday 17 May 2002 00:54, Andreas Dilger wrote:
> A minor question is whether to cap it at 65536 blocks/group or 65528?
> (The number of blocks per group must be a multiple of 8).
> 
> The current layout is such that you will _always_ have at least 3
> blocks in use for each group.  However, if we implement Ted's
> "metagroup" layout (which puts all of a group's bitmaps/itable blocks
> in the first group of its block of group descriptors) then there could
> be cases where a group has no blocks in use, and the free count will
> overflow.
> 
> Having the upper limit at 65536 is aesthetically pleasing, and it aligns
> nicely with LVM (which allocates chunks in power-of-two sizes), but may
> preclude changing such a filesystem to the metagroup layout without a
> larger effort on the resizer's part.  I'll go with 65528 I guess.

I like 65536 as well, but it's easy to relax your slightly lower limit
later if the metagroup design changes, and would not require a compatibility
flag, while tightening it would be a major pain.

> Note that going to a metagroup layout would also grow the distance
> between the itable and possible blocks quadratically (the number of
> group descriptors that fit into a block also grows with blocksize),
> but at least it is not cubic growth.  That said, the metagroup layout
> is probably only useful for cases where you _know_ you want huge files
> (in the multi-GB range) and locality of blocks to the single inode block
> is irrelevant.

-- 
Daniel