Re: Balancing raid5 after adding another disk does not move/use any data on it

["Jakub Husák" <jakub@husak.pro>]

This is a great tool Hans!  This kind of overview should be a part of 
btrfs-progs.

Mine looks currently like this, I have a few more days to go with 
rebalancing :)

flags            num_stripes     physical      virtual
-----            -----------     --------      -------
DATA|RAID5                 3      5.29TiB      3.53TiB
DATA|RAID5                 4    980.00GiB    735.00GiB
SYSTEM|RAID1               2    128.00MiB     64.00MiB
METADATA|RAID1             2    314.00GiB    157.00GiB

Btw, I checked the other utils in your python-btrfs and it seems that 
they are, sadly, not installed with simple pip install, which would be 
great. Maybe it needs a few lines in setup.py (i'm not too familiar with 
python packaging)?


On 16. 03. 19 20:51, Hans van Kranenburg wrote:
> On 3/16/19 5:34 PM, Hans van Kranenburg wrote:
>> On 3/16/19 7:07 AM, Andrei Borzenkov wrote:
>>> [...]
>>> This thread actually made me wonder - is there any guarantee (or even
>>> tentative promise) about RAID stripe width from btrfs at all? Is it
>>> possible that RAID5 degrades to mirror by itself due to unfortunate
>>> space distribution?
>> For RAID5, minimum is two disks. So yes, if you add two disks and don't
>> forcibly rewrite all your data, it will happily start adding two-disk
>> RAID5 block groups if the other disks are full.
> Attached an example that shows a list of used physical and virtual space
> ordered by chunk type (== block group flags) and also num_stripes (how
> many disks (or, dev extents)) are used. The btrfs-usage-report does not
> add this level of detail. (But maybe it would be interesting to add, but
> then I would add it into the btrfs.fs_usage code...)
>
> For the RAID56 with a big mess of different block groups with different
> "horizontal size" this will be more interesting than what it shows here
> as test:
>
> -# ./chunks_stripes_report.py /
> flags            num_stripes     physical      virtual
> -----            -----------     --------      -------
> DATA                       1    759.00GiB    759.00GiB
> SYSTEM|DUP                 2     64.00MiB     32.00MiB
> METADATA|DUP               2      7.00GiB      3.50GiB
>
>
> Hans