Re: [RFC] BIG_BG vs extended META_BG in ext4

[Mingming Cao <cmm@us.ibm.com>]

On Mon, 2007-07-02 at 11:49 -0400, Theodore Tso wrote:
> 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.
>

I agree.

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

How about incorporating some of the chunkfs ideas into this BIG_BG or
extended metablockgroups? The original block group size (128MB) is
probably too small that would results in many continous inodes. By
enlarging the size of groups via BIG_BG or extended metablockgroups, we
could add dirty/clean bit to allow partical/parallel fsck, and something
like that. Any thoughts on thhis?


Mingming