filesystem full when it's not? out of inodes? huh?

["Brian J. Murrell" <brian@interlinx.bc.ca>]

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I have a 5G /usr btrfs filesystem on a 3.0.0-12-generic kernel that is
returning ENOSPC when it's only 75% full:

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/rootvol-mint_usr
                      5.0G  2.8G  967M  75% /usr

And yet I can't even unpack a linux-headers package on to it, which
should be nowhere near 967MB.  dpkg says it will need 10MB:

$ sudo apt-get install -f
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following extra packages will be installed:
  linux-headers-3.0.0-16-generic
The following NEW packages will be installed:
  linux-headers-3.0.0-16-generic
0 upgraded, 1 newly installed, 0 to remove and 2 not upgraded.
264 not fully installed or removed.
Need to get 0 B/851 kB of archives.
After this operation, 10.8 MB of additional disk space will be used.
Do you want to continue [Y/n]? y
(Reading database ... 180246 files and directories currently installed.)
Unpacking linux-headers-3.0.0-16-generic (from .../linux-headers-3.0.0-16-generic_3.0.0-16.28_i386.deb) ...
dpkg: error processing /var/cache/apt/archives/linux-headers-3.0.0-16-generic_3.0.0-16.28_i386.deb (--unpack):
 unable to install new version of `/usr/src/linux-headers-3.0.0-16-generic/include/config/dvb/tuner/dib0070.h': No space left on device

And indeed, using dd I am able to create a 967MB file:

$ sudo dd if=/dev/zero of=/usr/bigfile bs=1M count=10000000
dd: writing `/usr/bigfile': No space left on device
967+0 records in
966+0 records out
1012924416 bytes (1.0 GB) copied, 16.1545 s, 62.7 MB/s

strace yields this as the cause of the ENOSPC:

8213  rename("/usr/src/linux-headers-3.0.0-16-generic/include/config/dvb/tuner/dib0070.h.dpkg-new", "/usr/src/linux-headers-3.0.0-16-generic/include/config/dvb/tuner/dib0070.h" <unfinished ...>
...
8213  <... rename resumed> )            = -1 ENOSPC (No space left on device)

So this starts to feel like some kind of inode count limitation.  But I
didn't think btrfs had inode count limitations.  Here's the df stats on
the filesystem:

$ btrfs filesystem df /usr
Data: total=3.22GB, used=3.22GB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=896.00MB, used=251.62MB
Metadata: total=8.00MB, used=0.00

I don't know if that's useful or not.

Any ideas?

Cheers
b.


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