Re: How does "btrfs fi show" show full?

[Duncan <1i5t5.duncan@cox.net>]

On Wed, 7 May 2014 04:30:30 -0700
Marc MERLIN <marc@merlins.org> wrote:

> > -dusage=85 balances all chunks that up to 85% full. The higher the
> > number, the more work that needs to be done.  
> 
> Aah, right. I see why it's more work. =20 only makes is process the
> few chunks that are up to 20% full which won't be many if your FS
> is almost full.

It's actually even less work than you imply.

Balance only has to rewrite the actual content, not the empty space in
the chunk.  So 20% full means it's only writing 20% of the
(possible/full) content, thus only taking 20% of the time to rewrite
that chunk that it'd take to rewrite a full chunk.

Which is why a usage=5 or 20 goes so fast, even if the system's
actually mostly empty but is all allocated.  With a 20% full chunk it's
rewriting five chunks into one; at 5%, it's rewriting 20 chunks into
one.  That goes pretty fast, even if there's a bunch of them to write!

-- 
Duncan - No HTML messages please, as they are filtered as spam.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman