Hello,
I've been a btrfs user for quite a number of years now, but it seems I need
the wiseness of the btrfs gurus on this one!
I have a 5-hdd btrfs raid1 setup with 4x3T+1x10T drives.
A few days ago, I replaced one of the 3T by a new 10T, running btrfs
replace and then resizing the FS to use all the available space of the new
device.
The filesystem was 90% full before I expanded it so, as expected, most of
the space on the new device wasn't actually allocatable in raid1, as very
few available space was available on the 4 other devs.
Of course the solution is to run a balance, but as the filesystem is now
quite big, I'd like to avoid running a full rebalance. This would be quite
i/o intensive, would be running for several days, and putting and
unecessary stress on the drives. This also seems excessive as in theory
only some Tb would need to be moved: if I'm correct, only one of two block
groups of a sufficient amount of chunks to be moved to the new device so
that the sum of the amount of available space on the 4 preexisting devices
would at least equal the available space on the new device, ~7Tb instead of
moving ~22T.
I don't need to have a perfectly balanced FS, I just want all the space to
be allocatable.
I tried using the -ddevid option but it only instructs btrfs to work on the
block groups allocated on said device, as it happens, it tends to move data
between the 4 preexisting devices and doesn't fix my problem. A full
balance with -dlimit=100 did no better.
Is there a way to ask the block group allocator to prefer writing to a
specific device during a balance? Something like -ddestdevid=N? This would
just be a hint to the allocator and the usual constraints would always
apply (and prevail over the hint when needed).
Or is there any obvious solution I'm completely missing?
Thanks,
Stéphane.
