Re: How to reliably measure fs usage with reflinks enabled?

[Dave Chinner <david@fromorbit.com>]

On Mon, May 14, 2018 at 05:02:53PM -0500, Eric Sandeen wrote:
> 
> 
> On 5/14/18 3:02 PM, Tarik Ceylan wrote:
> > How can one reliably measure filesystem usage on partitions that were compiled with -m reflink=1 ?
> > Here are some numbers i am measuring with df -h (on different partitions holding the same data):
> > 7.7G of 36G  (-b size=512  -m crc=0 )
> > 8.6G of 36G  (-b size=4096 -m crc=1 )
> 
> 8x larger inodes will take 8x more space, but you didn't say how many
> inodes you have allocated.
> 
> > 11G  of 36G  (-b size=1024 -m crc=1,reflink=1,rmapbt=1 -i sparse=1 )
> > 32G  of 864G (-b size=4096 -m crc=1,reflink=1 )
> 
> In that last case, you have a wildly different total fs size, so probably
> no fair comparison here either.
> 
> The reverse mapping btree also takes up space.  You're turning too many
> knobs at once.  ;)

Also, we reserve a lot of space for reflink/rmapbt metadata that
isn't actually used, so you're not actually using any more space
than the "-b size=4096 -m crc=1" case. I have plans for hiding that
reservation from users so that we don't get questions like this....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com