Re: Understanding "Used" in df

A L
2020-07-22T19:07:06Z

On 2020-07-22 17:10, Martin Steigerwald wrote:
> I have:
>
> % LANG=en df -hT /home
> Filesystem Type Size Used Avail Use% Mounted on
> /dev/mapper/sata-home btrfs 300G 175G 123G 59% /home
>
> And:
>
> merkaba:~> btrfs fi sh /home
> Label: 'home' uuid: […]
> Total devices 2 FS bytes used 173.91GiB
> devid 1 size 300.00GiB used 223.03GiB path /dev/mapper/sata-home
> devid 2 size 300.00GiB used 223.03GiB path /dev/mapper/msata-home
>
> merkaba:~> btrfs fi df /home
> Data, RAID1: total=218.00GiB, used=171.98GiB
> System, RAID1: total=32.00MiB, used=64.00KiB
> Metadata, RAID1: total=5.00GiB, used=1.94GiB
> GlobalReserve, single: total=490.48MiB, used=0.00B
>
> As well as:
>
> merkaba:~> btrfs fi usage -T /home
> Overall:
> Device size: 600.00GiB
> Device allocated: 446.06GiB
> Device unallocated: 153.94GiB
> Device missing: 0.00B
> Used: 347.82GiB
> Free (estimated): 123.00GiB (min: 123.00GiB)
> Data ratio: 2.00
> Metadata ratio: 2.00
> Global reserve: 490.45MiB (used: 0.00B)
> Multiple profiles: no
>
> Data Metadata System
> Id Path RAID1 RAID1 RAID1 Unallocated
> -- ---------------------- --------- -------- -------- -----------
> 1 /dev/mapper/sata-home 218.00GiB 5.00GiB 32.00MiB 76.97GiB
> 2 /dev/mapper/msata-home 218.00GiB 5.00GiB 32.00MiB 76.97GiB
> -- ---------------------- --------- -------- -------- -----------
> Total 218.00GiB 5.00GiB 32.00MiB 153.94GiB
> Used 171.97GiB 1.94GiB 64.00KiB
>
>
> I think I understand all of it, including just 123G instead of
> 300 - 175 = 125 GiB "Avail" in df -hT.
>
> But why 175 GiB "Used" in 'df -hT' when just 173.91GiB (see 'btrfs fi sh')
> is allocated *within* the block group / chunks?
>
> Does this have something to do with that global reserve thing?
>
> Thank you,
Hi,

I do not think global reserve should be counted in 'df' output, but it
does count all unallocated space, even if some of that space will be
used up by metadata chunks. Isn't the discrepancy the metadata allocated
vs used?

I made a post a little while ago on this subject, on how to improve the
space calculations if any one if interested in picking this up? I
believe the space calculation is wrong in all of the df/btrfs df/btrfs
us tools.