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

[Marc MERLIN <marc@merlins.org>]

On Wed, May 07, 2014 at 12:53:11PM +0200, Brendan Hide wrote:
> On 2014/05/07 09:59 AM, Marc MERLIN wrote:
> >[snip]
> >
> >Did I get this right?
> >I'm not sure I did, since it seems the bigger the -dusage number, the
> >more work balance has to do.
> >
> >If I asked -dsuage=85, it would do all chunks that are more than 15%
> >full?
> 
> -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.


> Doing a simulation with randomly-semi-filled chunks, "df" at 55%,
> and chunk utilisation at 86%, -dusage=55 balances 30% of the chunks,
> almost perfectly bringing chunk utilisation down to 56%. In my

Oh good, so my guess of using the same number for FS used and dusage=
was good, glad to know it worked well for you too.

> Pathological use-cases still apply however (for example if all
> chunks except one are exactly 54% full). The up-side is that if the
> algorithm is applied regularly (as in scripted and scheduled) then
> the situation will always be that the majority of chunks are going
> to be relatively full, avoiding the pathological use-case.

Sounds good. Also, the filesystem itself should ideally do this in the
background soon. What we're doing here is just a stopgap.

Marc
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