Re: [PATCH] ext4: don't load the block bitmap for block groups which have no space

[Theodore Ts'o <tytso@mit.edu>]

On Mon, Aug 13, 2012 at 11:02:08AM -0500, Eric Sandeen wrote:
> 
> Looks ok to me; I think this just further optimizes what was done
> in
> 
> 8a57d9d61a6e361c7bb159dda797672c1df1a691
> ext4: check for a good block group before loading buddy pages
> 
> correct?

Yes, that's right; it's a further optimization.

I can think of an additional optimization where if we are reading the
block bitmap for block group N, and the block bitmap for block group
N+1 hasn't been read before (so we don't have buddy bitmap stats), and
the block bitmap for bg N+1 is adjacent for bg N, we should read both
at the same time.  (And this could be generalized for N+2, N+3, etc.)

I'm not entirely sure whether it's worth the effort, but I suspect for
very full file systems, it might be very well be.  This is a more
general case of the problem where most people only benchmark mostly
empty file systems, and my experience has been that above 70-80%
utilization, our performance starts to fall off.  And while disk space
is cheap, it's not _that_ cheap, and there are always customers who
insist on using file systems up to a utilization of 99%, and expect
the same performance as when the file system was freshly formated.  :-(

	    	 	     	     	 - Ted