Re: Cannot balance FS (No space left on device)

[Hans van Kranenburg <hans.van.kranenburg@mendix.com>]

On 06/11/2016 12:10 AM, ojab // wrote:
> On Fri, Jun 10, 2016 at 9:56 PM, Hans van Kranenburg
> <hans.van.kranenburg@mendix.com> wrote:
>> You can work around it by either adding two disks (like Henk said), or by
>> temporarily converting some chunks to single. Just enough to get some free
>> space on the first two disks to get a balance going that can fill the third
>> one. You don't have to convert all of your data or metadata to single!
>>
>> Something like:
>>
>> btrfs balance start -v -dconvert=single,limit=10 /mnt/xxx/
>
> Unfortunately it fails even if I set limit=1:
>> $ sudo btrfs balance start -v -dconvert=single,limit=1 /mnt/xxx/
>> Dumping filters: flags 0x1, state 0x0, force is off
>>   DATA (flags 0x120): converting, target=281474976710656, soft is off, limit=1
>> ERROR: error during balancing '/mnt/xxx/': No space left on device
>> There may be more info in syslog - try dmesg | tail

Ah, apparently the balance operation *always* wants to allocate some new 
empty space before starting to look more close at the task you give it...

This means that it's trying to allocate a new set of RAID0 chunks 
first... and that's exactly the opposite of what we want to accomplish here.

If you really can add only one extra device now, there's always a more 
dirty way to get the job done.

What you can do for example is:
- partition the new disk in two partitions
- add them both to the filesystem (btrfs doesn't know both block devices 
are on the same physical disk, ghehe)
- convert a small number of data blocks to single
- then device delete the third disk again so the single chunks move back 
to the two first disks
- add the third disk back as one whole block device
- etc...

:D

Moo,

-- 
Hans van Kranenburg - System / Network Engineer
T +31 (0)10 2760434 | hans.van.kranenburg@mendix.com | www.mendix.com