Re: Please review and comment, dealing with btrfs full issues

[Marc MERLIN <marc@merlins.org>]

On Mon, May 05, 2014 at 07:07:29PM +0200, Brendan Hide wrote:
> "In the case above, because the filesystem is only 55% full, I can
> ask balance to rewrite all chunks that are more than 55% full:
> 
> legolas:~# btrfs balance start -dusage=50 /mnt/btrfs_pool1"
> 
> "-dusage=50" will balance all chunks that are 50% *or less* used,

Sorry, I actually meant to write 55 there.

> not more. The idea is that full chunks are better left alone while
> emptyish chunks are bundled together to make new full chunks,
> leaving big open areas for new chunks. Your process is good however
> - just the explanation that needs the tweak. :)

Mmmh, so if I'm 55% full, should I actually use -dusage=45 or 55?

> In your last example, a full rebalance is not necessary. If you want
> to clear all unnecessary chunks you can run the balance with
> -dusage=80 (636GB/800GB~=79%). That will cause a rebalance only of
> the data chunks that are 80% and less used, which would by necessity
> get about ~160GB worth chunks back out of data and available for
> re-use.

So in my case when I hit that case, I had to use dusage=0 to recover.
Anything above that just didn't work.

On Mon, May 05, 2014 at 07:09:22PM +0200, Brendan Hide wrote:
> Forgot this part: Also in your last example, you used "-dusage=0"
> and it balanced 91 chunks. That means you had 91 empty or
> very-close-to-empty chunks. ;)

Correct. That FS was very mis-balanced.

On Mon, May 05, 2014 at 02:36:09PM -0400, Calvin Walton wrote:
> The standard response on the mailing list for this issue is to
> temporarily add an additional device to the filesystem (even e.g. a 4GB
> USB flash drive is often enough) - this will add space to allocate a few
> new chunks, allowing the balance to proceed. You can remove the extra
> device after the balance completes.

I just added that tip, thank you.
 
On Tue, May 06, 2014 at 02:41:16PM +1000, Russell Coker wrote:
> Recently kernel 3.14 allowed fixing a metadata space error that seemed to be 
> impossible to solve with 3.13.  So it's possible that some of my other 
> problems with a lack of metadata space could have been solved with kernel 3.14 
> too.

Good point. I added that tip too.

Thanks,
Marc
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