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

[Eric Sandeen <sandeen@redhat.com>]

On 8/13/12 1:49 PM, Theodore Ts'o wrote:
> 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.  :-(

I did some tests w/ very large filesystems, fallocating 1T at a time until
full.  ext4 tended to fall down pretty badly towards the end.  Anything that
can reduce the time it takes to find free blocks as a very large filesystem
fills would probably be useful....

-eric

> 	    	 	     	     	 - Ted
>