Re: [PATCH v2 1/3] Btrfs: get more accurate output in df command.

[Duncan <1i5t5.duncan@cox.net>]

Goffredo Baroncelli posted on Fri, 12 Dec 2014 19:00:20 +0100 as
excerpted:

> $ sudo ./btrfs fi df /mnt/btrfs1/
> Data, RAID1: total=1.00GiB, used=512.00KiB
> Data, single: total=8.00MiB, used=0.00B
> System, RAID1: total=8.00MiB, used=16.00KiB
> System, single: total=4.00MiB, used=0.00B
> Metadata, RAID1: total=1.00GiB, used=112.00KiB
> Metadata, single: total=8.00MiB, used=0.00B
> GlobalReserve, single: total=16.00MiB, used=0.00B
> 
> In this case the filesystem is empty (it was a new filesystem !).
> However a 1G metadata chunk was already allocated. This is the reasons
> why the free space is only 4Gb.

Trivial(?) correction.

Metadata chunks are quarter-gig, not 1 gig.  So that's 4 quarter-gig 
metadata chunks allocated, not a (one/single) 1-gig metadata chunk.

> On my system the ratio metadata/data is 234MB/8.82GB = ~3%, so ignoring
> the metadata chunk from the free space may not be a big problem. 

Presumably your use-case is primarily reasonably large files; too large 
for their data to be tucked directly into metadata instead of allocating 
an extent from a data chunk.

That's not always going to be the case.  And given the multi-device 
default allocation of raid1 metadata, single data, files small enough to 
fit into metadata have a default size effect double their actual size!  
(Tho it can be noted that given btrfs' 4 KiB standard block size, without 
metadata packing there'd still be an outsized effect for files smaller 
than half that, 2 KiB or under, but there it'd be in data chunks, not 
metadata.)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
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