Add btrfs post.

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+I'm currently migrating my netbook's SSD to [btrfs][] to save space
+and increase corruption checking.  Here are my notes on what I did.
+
+The netbook uses a USB stick to store its entire file system.  Small
+and slow, but cheap (actually, this USB stick was free as in beer ;).
+I initially tried to convert to btrfs on the fly, but I'd previously
+had it running [ext2][] with 1 [KiB][] blocks, and `btrfs-convert`
+didn't like the small blocks.
+
+So we'll have to create a new filesystem from scratch.  Plug the USB
+stick into a real computer to rework the drive, and copy the data off
+the old partition:
+
+    # mkdir /mnt/btrfs
+    # mount /dev/sdb2 /mnt/btrfs
+    # mkdir /tmp/old
+    # rsync -av /mnt/btrfs/ /tmp/old/
+
+Unmount and create a new file system on your partition:
+
+    # umount /dev/sdb2
+    # mkbtrfs -L GENTOO -m single -d single /dev/sdb2
+
+Note that I've disabled raid for both the metadata and data to save
+space.  This will make it harder to recover from corruption, but the
+checksumming will still detect the *presence* of corruption.  Now,
+mount the new filesystem:
+
+    # mount -t btrfs -o compress,compress-force,ssd,noacl /dev/sdb2 /mnt/btrfs
+
+and populate `/mnt/btrfs` with the filesystem for the netbook:
+
+    # rsync -av /tmp/old/ /mnt/btrfs/
+
+I'm not sure exactly how much space I've saved (I forgot to check
+while I was still in ext2), and free space is a [tricky issue][space],
+but:
+
+    $ df -h /dev/sdb2
+    Filesystem            Size  Used Avail Use% Mounted on
+    /dev/sdb2             3.7G  1.3G  2.5G  34% /tmp/a
+    $ btrfs filesystem df /mnt/btrfs/
+    Data: total=1.47GB, used=951.89MB
+    Metadata: total=776.00MB, used=289.24MB
+    System: total=4.00MB, used=4.00KB
+    # btrfs filesystem show /dev/sdb2
+    Label: 'GENTOO'  uuid: 3c4ba41f-13ca-4549-b093-e0114e748d88
+            Total devices 1 FS bytes used 1.21GB
+            devid    1 size 3.66GB used 2.24GB path /dev/sdb2
+
+    Btrfs Btrfs v0.19
+
+So now there is plenty of breathing room.
+
+Besides checksumming and compression, I was also interested in
+[snapshots][].  Take a snapshot with
+
+    # btrfs subvolume snapshot /mnt/btrfs /mnt/btrfs/snapshot_name
+
+Mount the snapshot with
+
+    # mount -t btrfs -o subvol=snapshot_name,... /dev/sdb2 /mnt/btrfs
+
+[btrfs]: http://btrfs.wiki.kernel.org/
+[ext2]: http://en.wikipedia.org/wiki/Ext2
+[KiB]: http://en.wikipedia.org/wiki/Kibibyte
+[space]: https://btrfs.wiki.kernel.org/index.php/FAQ#Why_are_there_so_many_ways_to_check_the_amount_of_free_space.3F
+[snapshots]: https://btrfs.wiki.kernel.org/index.php/UseCases#Snapshots_and_subvolumes
+
+[[!tag tags/linux]]