Re: Have 15GB missing in btrfs filesystem.

[Marc MERLIN <marc@merlins.org>]

On Sat, Oct 27, 2018 at 02:12:02PM -0400, Remi Gauvin wrote:
> On 2018-10-27 01:42 PM, Marc MERLIN wrote:
> 
> > 
> > I've been using btrfs for a long time now but I've never had a
> > filesystem where I had 15GB apparently unusable (7%) after a balance.
> > 
> 
> The space isn't unusable.  It's just allocated.. (It's used in the sense
> that it's reserved for data chunks.).  Start writing data to the drive,
> and the data will fill that space before more gets allocated.. (Unless
> you are using an older kernel and the filesystem gets mounted with ssd
> option, in which case, you'll want to add nossd option to prevent that
> behaviour.)
> 
> You can use btrfs fi usage to display that more clearly.
 
Got it. I have disk space free alerts based on df, which I know doesn't
mean that much on btrfs. Maybe I'll just need to change that alert code
to make it btrfs aware.
 
> > I can try a defrag next, but since I have COW for snapshots, it's not
> > going to help much, correct?
> 
> The defrag will end up using more space, as the fragmented parts of
> files will get duplicated.  That being said, if you have the luxury to
> defrag *before* taking new snapshots, that would be the time to do it.

Thanks for confirming. Because I always have snapshots for btrfs
send/receive, defrag will duplicate as you say, but once the older
snapshots get freed up, the duplicate blocks should go away, correct?

Back to usage, thanks for pointing out that command:
saruman:/mnt/btrfs_pool1# btrfs fi usage .
Overall:
    Device size:		 228.67GiB
    Device allocated:		 203.54GiB
    Device unallocated:		  25.13GiB
    Device missing:		     0.00B
    Used:			 192.01GiB
    Free (estimated):		  32.44GiB	(min: 19.88GiB)
    Data ratio:			      1.00
    Metadata ratio:		      2.00
    Global reserve:		 512.00MiB	(used: 0.00B)

Data,single: Size:192.48GiB, Used:185.16GiB
   /dev/mapper/pool1	 192.48GiB

Metadata,DUP: Size:5.50GiB, Used:3.42GiB
   /dev/mapper/pool1	  11.00GiB

System,DUP: Size:32.00MiB, Used:48.00KiB
   /dev/mapper/pool1	  64.00MiB

Unallocated:
   /dev/mapper/pool1	  25.13GiB


I'm still seing that I'm using 192GB, but 203GB allocated.
Do I have 25GB usable:
    Device unallocated:		  25.13GiB

Or 35GB usable?
    Device size:		 228.67GiB
      -
    Used:                        192.01GiB
      = 36GB ?    

Yes I know that I shouldn't get close to filling up the device, just
trying to clear up if I should stay below 25GB or below 35GB

Thanks,
Marc
-- 
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Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
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