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I have a btrfs partition. When I run df -h, it shows:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda2       113G  101G  8.3G  93% /home

From Why is that? Is it because reserved space for root as wth ext2/3/4? Or is it something else? If the former, how can I change it and reclaim those 4GB ?

As per btrfs wiki, I know that metadata are stored twice which inflates the size of "Used" data:

user@machine:~$ df -h /
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             894G  311G  583G  35% /
user@machine:~$ btrfs fi df /
Metadata: total=18.00GB, >>used=6.10GB<<  *2=  12.20GB
Data: total=358.00GB, >>used=298.37GB<<   *1= 298.37GB
System: total=12.00MB, >>used=40.00KB<<   *1=   0.00GB
                                           == 310.57GB 
                                           ~~ 311   GB

But this still does not explain why Used + Avail < Size.

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btrfs.wiki.kernel.org/articles/f/a/q/FAQ_1fe9.html - read the "Why are there so many ways to check the amount of free space?" section, it has lots of information. –  Mat Apr 28 '12 at 6:10
Yes, it has plenty of informations, but in all their examples Used + Avail = Size in df output. I have updated the question with this. –  sup Apr 28 '12 at 8:32
Does lsof show any deleted files? (The btrfs output from your system would have been a bit more appropriate than that from the wiki.) –  Mat Apr 28 '12 at 16:53
I know, but I was lazy to reformat it and it is analogical. If lsof |grep 'DEL'|wc -l gives the number of deleted files, it is 338. They do not seem big though (can I get their overall size somehow?) –  sup Apr 28 '12 at 18:31
@Mat Here's a working link to the BTRFS FAQ: Why are there so many ways to check the amount of free space? –  ali_m Nov 3 at 14:39

1 Answer 1

up vote 4 down vote accepted

Unless you specified otherwise when you formatted, the default is to store duplicate copies of the metadata blocks for improved reliability. You probably have 2gb worth of metadata that is stored twice, using 4gb. You can see more details with btrfs filesystem df.

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btrfs filesystem df /home gives Metadata, DUP: total=1.75GB, used=385.70MB so I have about 771MB of Metadata stored, together with Data it amounts to 101G, but it does not explain why Used + Avail < Size - I expanded the question to reflect this. –  sup Apr 28 '12 at 8:33
@sup, 1.75GB is allocated for metadata, so it consumes twice that or 3.5GB of space. Only 385mb of that 1.75 gb is currently used for metadata, but the full 1.75GB is reserved for that use and so is not counted towards available space for file data. –  psusi Apr 28 '12 at 20:29
Ah, now I understand. I guess the examples from btrfs wiki just use some old df that did not know about this. Thanks for the answer! –  sup Apr 28 '12 at 22:36

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