Re: Balance on 5-disk RAID1 put all data on 2 disks, leaving the rest empty

[Pedro Macedo <pmacedo@pmacedo.com>]

On 27.10.23 06:21, Anand Jain wrote:
> On 10/26/23 05:15, Roman Mamedov wrote:
>> On Wed, 25 Oct 2023 17:08:08 -0400
>> Remi Gauvin <remi@georgianit.com> wrote:
>>
>>> On 2023-10-25 4:29 p.m., Peter Wedder wrote:
>>>> Hello,
>>>>
>>>> I had a RAID1 array on top of 4x4TB drives. Recently I removed one 
>>>> 4TB drive and added two 16TB drives to it. After running a full, 
>>>> unfiltered balance on the array, I am left in a situation where all 
>>>> the 4TB drives are completely empty, and all the data and metadata 
>>>> is on the 16TB drives. Is this normal? I was expecting to have at 
>>>> least some data on the smaller drives.
>>>>
>>>
>>> Yes, this is normal.  The BTRFS allocates space in drives with the the
>>> most available free space.  The idea is to balance the 'unallocated'
>>> space on each drive, so they can be filled evenly.  The 4TB drives will
>>> be used when the 16TB dives have less than 4TB unallocated.
>>
>
> Correct. That's the only allocation method we have at the moment. Do you
> have any feedback on whether there are any other allocation methods that
> make sense?


IMHO, based on the frequency of this question appearing here/on 
reddit/other sites, perhaps allocation by absolute space used?  It 
should fit the expectations of most folks that if you have free space on 
a disk it will be utilised, plus has potential performance implications 
by always using as many devices as possible to write to as long as they 
have any space left.

Regards,

Pedro


>
> Thanks, Anand
>
>> Interesting question and resolution. I'd be surprised by that as well.
>>
>> Now, a great chance to "btrfs dev delete" all three remaining 4TB 
>> drives and
>> unplug them for the time being, to save on noise, heat and power 
>> consumption!
>