Re: [RFC] BIG_BG vs extended META_BG in ext4

[Theodore Tso <tytso@mit.edu>]

On Sun, Jul 01, 2007 at 09:48:33AM -0500, Jose R. Santos wrote:
> Is your concern due to being unable to find contiguous block in the
> case that a bad disk area is in one of the bitmap blocks?  One thing we
> can do is try to search for another set of contiguous blocks and if we
> fail to find one, we can flag the block group and move to an indirect
> block approach to allocating the bitmaps.  At this point, we do lose
> some of the performance benefits of BIG_BG, but we would still be able
> to use the block group.

Yes, my concern is what we might need to do if for some reason e2fsck
needs to reallocate the bitmap blocks.  I don't think an indirect
block scheme is the right approach, though; we're adding a lot of
complexity for a case that probably wouldn't be used but very, very
rarely.

My proposal (as we discsused) in the call, is to implement BIG_BG as
meaning the following:

	1) Implementations must understand and use the s_desc_size
	superblock field to determine whether block group descriptors
	are the old 32 bytes or the newer 64 bytes format.  
	
	2) Implementations must support the newer ext4_group_desc
	format in particular to support bg_free_blocks_count_hi and
	bg_free_inodes_count_hi

	3) Implementations will relax constraints on where the
	superblock, bitmaps, and inode tables for a particular block
	group will be stored.

So with that, we can experiment with what size block groups really
make sense, versus using the extended metablockgroup idea, or possibly
doing both.

						- Ted