On 12/01/11 14:02, Hugo Mills wrote:
I've had a go at determining exactly what happens when you create a
filesystem without enough devices to meet the requested replication
strategy:
Thanks - being new to this I haven't set up the infrastructure to try
these tests - but am interested because (as I said before) its important
for it to work if I want to migrate my mdmadm/lvm raid setup to btrfs.
# 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.
Is it a bug? or intentional design?
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
Where does this /dev/sr0 come from? I don't see it referenced elsewhere
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.
You don't appear to have copied any data on to it whilst in degraded
mode, to see if it behaves like the initial case and reverts to RAID0
when you copy data. You only copied data whilst both devices where
operational.
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
