On 2016-06-12 06:35, boli wrote:
It has now been doing "btrfs device delete missing /mnt" for about 90 hours.
These 90 hours seem like a rather long time, given that a rebalance/convert
from 4-disk-raid5 to 4-disk-raid1 took about 20 hours months ago, and a scrub
takes about 7 hours (4-disk-raid1).
OTOH the filesystem will be rather full with only 3 of 4 disks available, so I do expect
it to take somewhat "longer than usual".
Would anyone venture a guess as to how long it might take?
It's done now, and took close to 99 hours to rebalance 8.1 TB of data from a
4x6TB raid1 (12 TB capacity) with 1 drive missing onto the remaining 3x6TB
raid1 (9 TB capacity).
Now I made sure quotas were off, then started a screen to fill the new 8 TB
disk with zeros, detached it and and checked iotop to get a rough estimate on
how long it will take (I'm aware it will become slower in time).
After that I'll add this 8 TB disk to the btrfs raid1 (for yet another
rebalance).
The next 3 disks will be replaced with "btrfs replace", so only one rebalance
each is needed.
I assume each "btrfs replace" would do a full rebalance, and thus assign chunks
according to the normal strategy of choosing the two drives with the most free space,
which in this case would be a chunk to the new drive, and a mirrored chunk to that
existing 3 drive with most free space.
Replace doesn't need to do a balance, it's largely just a block level
copy of the device being replaced, but with some special handling so
that the filesystem is consistent throughout the whole operation. This
is most of why it's so much more efficient than add/delete.
What I'm wondering is this:
If the goal is to replace 4x 6TB drive (raid1) with 4x 8TB drive (still raid1),
is there a way to remove one 6 TB drive at a time, recreate its exact contents
from the other 3 drives onto a new 8 TB drive, without doing a full rebalance?
That is: without writing any substantial amount of data onto the remaining 3
drives.
The most efficient way of converting the array online without adding any
more disks than you have to begin with is:
1. Delete one device from the array with device delete.
2. Physically switch the now unused device with one of the new devices.
3. Use btrfs replace to replace one of the devices in the array with the
newly connected device (and make sure to resize to the full size of the
new device).
4. Repeat from step 2 until you aren't using any of the old devices in
the array.
5. You should have one old device left unused, physically switch it for
a new device.
6. Use btrfs device add to add the new device to the array, then run a
full balance.
This will result in only two balances being needed (one implicit in the
device delete, and the explicit final one to restripe across the full
array), and will result in the absolute minimum possible data transfer.
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