Hello,
I am wondering if the calculations for disk space can be improved?
If we look at my current root filesystem, the available space is
reported as 163GiB, but I think it would be more reasonable to report
this as 158GiB. This is because we have to deduct any metadata
allocation what would happen when filling up the filesystem.
df and btrfs fi usage numbers:
==============
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 234G 71G 163G 31% /
# btrfs fi us /
Overall:
Device size: 233.47GiB
Device allocated: 75.03GiB
Device unallocated: 158.44GiB
Device missing: 0.00B
Used: 70.36GiB
Free (estimated): 162.55GiB (min: 162.55GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 192.73MiB (used: 0.00B)
Multiple profiles: no
Data,single: Size:73.00GiB, Used:68.88GiB (94.36%)
/dev/sda3 73.00GiB
Metadata,single: Size:2.00GiB, Used:1.47GiB (73.63%)
/dev/sda3 2.00GiB
System,single: Size:32.00MiB, Used:16.00KiB (0.05%)
/dev/sda3 32.00MiB
Unallocated:
/dev/sda3 158.44GiB
==============
If we follow the numbers:
* Data / Metadata ratio = 68.88/1.47 = 46.54.
* Estimated metadata usage when full: Unallocated + (73.00-68.88) /
46.54 + 1,47 = 4.96, round up to 5.00 GiB
So I think that df's "Avail:" and btrfs fi usage's "Free (estimated):"
should be:
* Estimated available user data space: 158.44 + (73.00-68.88) - 5.00 ~
157,56GiB
This calculation is probably harder with multiple devices. But I think
we can use same principle of estimating based on current allocation
ratios between data and metadata.
~ Anders
