Into my unRaid I have all disks from Array formated as BTRFS. For my expansion with 3 WD Red 4TB (2 for Parity and 1 for array) I started to clean other 3 smaller disks.

At this moment one of the 3 disks is completely empty, there are 2 more to go.

I have noticed that even with no data on Disk1, the disk have 538MB used.

 

 

Quote

root@Node804:/# df -h /mnt/disk1
Filesystem      Size  Used Avail Use% Mounted on
/dev/md1        466G  513M  465G   1% /mnt/disk1
 

root@Node804:/# btrfs fi df /mnt/disk1/
Data, single: total=1.00GiB, used=512.00KiB
System, single: total=4.00MiB, used=16.00KiB
Metadata, single: total=1.00GiB, used=112.00KiB
GlobalReserve, single: total=512.00MiB, used=0.00B
 

root@Node804:/# du -sh /mnt/disk1
16K     /mnt/disk1
 

root@Node804:/# ls -la /mnt/disk1
total 16
drwxrwxrwx  1 nobody users   0 Jul  4 21:35 ./
drwxr-xr-x 14 root   root  280 Jul  4 12:38 ../

 

root@Node804:/#

 

Can someone explain me why this is happening?

It is quite normal for a disk that has no files on it to have a small amount of space used.    Even an empty disk has space used for the control structures that make up the file system.

Hi,

I think you've got what I'll call 'stuck deletes' - typically the OS will sometimes allow you to 'delete' a file but it doesn't actually remove the file(s) until the process that's holding them open exits.

It's easy enough to check:

lsof | grep -i deleted

 

This should show you the files that are marked as 'deleted' but are essentially pending deletion. It'll also show the process that's holding them open - you could try turning that process off and on again to fix it. Failing that - array off then on, or finally a restart should clear this issue up.

While I'd expect to see some disk usage - 500MB is bit higher than one would expect for 'the control structures that make up the file system.'

Del

9 minutes ago, Delarius said:

It's easy enough to check:

lsof | grep -i deleted

Quote

root@Node804:~# lsof | grep -i deleted|grep disk1
root@Node804:~#
 

 

That is quite odd, often in similar cases I've seen 'deleted' files cause this space mismatch. Clearly not in this case unless it's got them open under the device /dev/md1 or /dev/sd[a-z]

Another thing you can check - and I'm not at my server right now, but there's a btrfs balance item somewhere in the unraid Web GUI - I seem to recall under disk settings?

I just noted that it's definitely allocating that space as GlobalReserve:

51 minutes ago, FlorinB said:

root@Node804:/# btrfs fi df /mnt/disk1/
Data, single: total=1.00GiB, used=512.00KiB
System, single: total=4.00MiB, used=16.00KiB
Metadata, single: total=1.00GiB, used=112.00KiB
GlobalReserve, single: total=512.00MiB, used=0.00B

I believe the filesystem has reserved that space but clearly it isn't necessary for a blank disk - if you can find that page in the GUI it should allow you to fix it that way, otherwise it's something like:

btrfs balance start -v -dusage=0 /mnt/disk1

7 minutes ago, Delarius said:

btrfs balance start -v -dusage=0 /mnt/disk1

Quote

root@Node804:~# btrfs balance start -v -dusage=0 /mnt/disk1
Dumping filters: flags 0x1, state 0x0, force is off
  DATA (flags 0x2): balancing, usage=0
Done, had to relocate 0 out of 3 chunks
root@Node804:~# df -h /mnt/disk1
Filesystem      Size  Used Avail Use% Mounted on
/dev/md1        466G  513M  465G   1% /mnt/disk1
root@Node804:~#

 

10 minutes ago, Delarius said:

I believe the filesystem has reserved that space but clearly it isn't necessary for a blank disk

Looks like you are right, btrfs has reserved that space, even if is not used

Quote

root@Node804:~# btrfs filesystem usage /mnt/disk1
Overall:
    Device size:                       465.76GiB
    Device allocated:              2.00GiB
    Device unallocated:         463.76GiB
    Device missing:                0.00B
    Used:                                  640.00KiB
    Free (estimated):             464.76GiB      (min: 464.76GiB)
    Data ratio:                        1.00
    Metadata ratio:               1.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,single: Size:1.00GiB, Used:512.00KiB
   /dev/md1        1.00GiB

Metadata,single: Size:1.00GiB, Used:112.00KiB
   /dev/md1        1.00GiB

System,single: Size:4.00MiB, Used:16.00KiB
   /dev/md1        4.00MiB

Unallocated:
   /dev/md1      463.76GiB

Similar info can be found here https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-filesystem

 

Thanks.