Linux – How to remove a drive from a BTRFS RAID 1 setup to make it just a single (BTRFS) drive

I found the following on the btrfs wiki, specifically the Gotchas page (quoted verbatim):

• on a -d raid1 volume, df will show total raw space, space used by data (not counting duplication), and the difference between those two numbers. This can be surprising since you'll run out of space before the Use% reaches 50. (up to and including 2.6.33 -- reporting changed in 2.6.34)

• In 2.6.34, df will show total raw space, space used by data (factoring in duplication for raid1), and raw free space, taking metadata and data into consideration. You can still hit an "out of space" condition well before free space reaches 0, especially if, under raid1, your allocated space is unbalanced, and the system finds it impossible to find space for your new write on two separate disks.

Since you're running Ubuntu 10.04, which uses Linux kernel version 2.6.32, it appears you're seeing the "surprising" behaviour described by the first bullet point. So, in all likelihood, the filesystem you created really is RAID1, df is just over-reporting its size.

Btrfs can use different raid levels for data and Metadata:

the default (even for one disk) is raid1 for the metadata (directories etc) and raid0 for the data.

If you did not change this then likely you will have no problem adding the second disc and running re-balance. because only the metadata will be copied to both discs (you can see your metadata size with btrfs filesystem df /). Just be aware that if either of your disks fails you loose data.

because the 2tb disk is sooooooooo much bigger than the 500g it would perhaps give you better odds if you add the new one and then remove the old one (the odds of one specific drive failing are a lot less than the odds of either of the drives failing).

if you plan on having a raid array later (with more similar sized drives) you may want to re-create the filesystem on the new drive with raid1 for both data and metadata and then copy everything over. then later when you have more money buy the second 2tb drive.

ps: using raid1 on a singl drive means the data will be stored in two locations on that one drive (to protect against corruption) and will reduce your storage space (its a really really good idea for the metadata).

pss: seriously, dont be tempted to not use raid1 for metadata. psss: there is a very good chance that btrfs will gain the ability to change raid levels dynamically.