I've had a go at determining exactly what happens when you create a filesystem without enough devices to meet the requested replication strategy:
# mkfs.btrfs -m raid1 -d raid1 /dev/vdb
# mount /dev/vdb /mnt
# btrfs fi df /mnt
Data: total=8.00MB, used=0.00
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=153.56MB, used=24.00KB
Metadata: total=8.00MB, used=0.00
The data section is single-copy-only; system and metadata are DUP.
This is good. Let's add some data:
# cp develop/linux-image-2.6.3* /mnt
# btrfs fi df /mnt
Data: total=315.19MB, used=250.58MB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=153.56MB, used=364.00KB
Metadata: total=8.00MB, used=0.00
Again, much as expected. Now, add in a second device, and balance:
# btrfs dev add /dev/vdc /mnt
# btrfs fi bal /mnt
# btrfs fi df /mnt
Data, RAID0: total=1.20GB, used=250.58MB
System, RAID1: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=128.00MB, used=308.00KB
This is bad, though. Data has reverted to RAID-0.
Now, just to check, what happens when we create a filesystem with
enough devices, fail one, and re-add it?
# mkfs.btrfs -d raid1 -m raid1 /dev/vdb /dev/vdc
# mount /dev/vdb /mnt
# # Copy some data into it
# btrfs fi df /mnt
Data, RAID1: total=1.50GB, used=1.24GB
Data: total=8.00MB, used=0.00
System, RAID1: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=307.19MB, used=1.80MB
Metadata: total=8.00MB, used=0.00
# umount /mnt
OK, so what happens if we fail one drive?
# dd if=/dev/zero of=/dev/vdb bs=1M count=16
# mount /dev/vdc /mnt -o degraded
# btrfs dev add /dev/vdd /mnt
# btrfs fi show
failed to read /dev/sr0
Label: none uuid: 2495fe15-174f-4aaa-8317-c2cfb4dade1f
Total devices 3 FS bytes used 1.25GB
devid 2 size 3.00GB used 1.81GB path /dev/vdc
devid 3 size 3.00GB used 0.00 path /dev/vdd
*** Some devices missing
Btrfs Btrfs v0.19
# btrfs fi bal /mnt
# btrfs fi df /mnt
Data, RAID1: total=1.50GB, used=1.24GB
System, RAID1: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=128.00MB, used=1.41MB
This looks all well and good. So it looks like it's just the
"create-in-degraded-mode" idea that doesn't work.
Kernel is btrfs-unstable, up to 65e5341b (plus my balance-progress
patches, but those shouldn't affect this).
Hugo.
PS. I haven't tried with RAID-10 yet, but I suspect that it'll be much
the same.
--
=== Hugo Mills: h...@... carfax.org.uk | darksatanic.net | lug.org.uk ===
PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
--- "You are demons, and I am in Hell!" "Well, technically, it's ---
London, but it's an easy mistake to make."
signature.asc
Description: Digital signature
