Btrfs storage advice

Hi btrfs list,
I am looking for some counsel regarding how to best (and most safely) utilize extra space on my btrfs installation. I set up a btrfs installation about 6 months ago. I wanted to test the system while waiting for mainline acceptance and support. The machine being used has 13 1Tb drives. 12 as a btrfs collection (stripe data, mirror metadata) and 1 ext4 as a system drive. We are running kernel 3.2.0-rc4. I know that it is not the latest, but it has been extremely stable for our needs. Currently the system holds backup files. 2 other filesystems are nfs mounts on the machine and backups are created by rsyncing these mounts onto btrfs. The btrfs copies are also snapshotted, so 2 copies exist of backup data. I have added the output of btrfs fi show and btrfs fi df below so you can see the layout, as well as a standard df -h. As will be readily apparent, my nfs disks are approaching storage limits. Due to financial constraints I must use the space on the btrfs system for nfs storage. My first thought is to take 3 or 4 T as a subvol and export it as nfs. I have not heard of anyone else exporting btrfs, is it possible? Next idea is to split several drives off the btrfs system. I have removed drives and replaced them as experiments with the fs but had much less data on them when I was trying that. I have read many times on the list, about size issues with btrfs, and filesystems reporting full when they were far from it. As my system has been very stable just r/w data and creating and removing subvols, I am reluctant to change the disk layout, but we will do what we have to. Also, if I split disks out they could be mirrored, like our other nfs systems. However, I can stand a small amount of filesystem downtime. Therefore to maximize space we may look at not mirroring the segment but just mount a backup snapshot if a main fs drive goes out. Final question is what about backup space. Regardless of how I structure the new storage segment, it will need to be backed up with the rest of the system. Once again, I am between maximizing available storage and leaving breathing room for btrfs. As I currently backup over 4T on btrfs perhaps I should only allocate 2T for new storage thus creating 2T storage, 6+T backup and 1+T breathing room. I am not in a panic situation, but I will need to create the new storage over the next 2 months. I would really appreciate any feed back and comments concerning this operation. Thanks in advance. Jim Maloney

[root@btrfs ~]# btrfs fi show
failed to read /dev/sr0
Label: none uuid: c21f1221-a224-4ba4-92e5-cdea0fa6d0f9

        Total devices 12 FS bytes used 4.62TB
        devid   12 size 930.99GB used 414.75GB path /dev/sdl
        devid   11 size 930.99GB used 414.75GB path /dev/sdk
        devid   10 size 930.99GB used 414.99GB path /dev/sdj
        devid    9 size 930.99GB used 414.99GB path /dev/sdi
        devid    5 size 930.99GB used 414.99GB path /dev/sde
        devid    2 size 930.99GB used 414.74GB path /dev/sdb
        devid    1 size 930.99GB used 414.76GB path /dev/sda
        devid    7 size 930.99GB used 414.99GB path /dev/sdg
        devid    3 size 930.99GB used 414.74GB path /dev/sdc
        devid    4 size 930.99GB used 414.74GB path /dev/sdd
        devid    6 size 930.99GB used 414.99GB path /dev/sdf
        devid    8 size 930.99GB used 414.99GB path /dev/sdh

[root@btrfs ~]# btrfs fi df /btrfs Data, RAID0: total=4.54TB, used=4.50TB
Data: total=8.00MB, used=0.00
System, RAID1: total=8.00MB, used=324.00KB
System: total=4.00MB, used=0.00 Metadata, RAID1: total=164.25GB, used=122.97GB
Metadata: total=8.00MB, used=0.00

[root@btrfs ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sdm2 196G 49G 138G 26% /

tmpfs                  16G     0   16G   0% /dev/shm
/dev/sdm1             2.0G  137M  1.8G   8% /boot
/dev/sdm5             1.2T   19G  1.1T   2% /var
/dev/sda               11T  4.8T  6.1T  44% /btrfs
10.2.0.42:/data/sites
                      2.6T  2.1T  388G  85% /nfs2/data/sites
10.2.0.40:/data/sites
                      2.6T  2.3T  218G  92% /nfs1/data/sites