I haven't converted from RAID 1 to RAID 5 in BTRFS, but I have done other BTRFS conversions, and according to the documentation they are all equally easy.
It is as simple as
btrfs balance start -dconvert=raid5 -mconvert=raid5 /mnt
It is not difficult, but it will take some time (many hours).
Yes, RAID5 is still marked as experimental. Per the most recent documentation I can find, scrub still doesn't work with RAID5, so any data errors can't be corrected.
One other approach to consider: with BTRFS RAID1, you can use three 3TB drives to get 4.5TB usable space. Yes, you can do mirroring with an odd number of drives. BTRFS doesn't mirror disks, it mirrors blocks. So, for example, some files will be mirrored on drives 1 and 2, some on 2 and 3, and some on 1 and 3.
You can even have different size drives with BTRFS RAID1. I have a set with one 2TB drive and three 1TB drives, giving 2.5TB usable space.
BTRFS RAID1 is really cool.
check your question. are you converting from raid0 or raid1? – Russell Uhl – 2015-04-13T17:38:04.133
as to your question, raid 5 introduces parity. Not only that, but it is DISTRIBUTED parity. That means a large amount of data needs to be moved around your disks. As for how easy the process is to actually perform, I can't help you there. – Russell Uhl – 2015-04-13T17:42:18.527
@RussellUhl sorry I fixed it. – Buttink – 2015-04-13T20:59:58.007
1lol tfw reading it after june 2016 when btrfs RAID5 implementation turned out to be fatally flawed and unable to rebuild array properly after disk failure... – Lapsio – 2016-09-19T11:24:13.700
@user2111737 Good thing I was to lazy and poor to buy a 3rd hard drive. – Buttink – 2016-09-19T13:46:30.447
I wasn't :| now I have a problem... – Lapsio – 2016-09-19T13:52:09.733
@user2111737 Converting from raid5 to raid1 should be safe, as long as your disks don't break down before the operation is completed. – pipe – 2016-09-26T03:53:54.653