[PATCH] btrfs-progs: add RAID5/6 support to btrfs fi us

[Goffredo Baroncelli <kreijack@libero.it>]

Hi all,

this patch adds support for the raid5/6 profiles in the command
'btrfs filesystem usage'.

Until now the problem was that the value r_{data,metadata}_used is not
easy to get for a RAID5/6, because it depends by the number of disks.
And in a filesystem it is possible to have several raid5/6 chunks with a
different number of disks.

In order to bypass this issue, I reworked the code to get rid of these
values where possible and to use the l_{data,metadata}_used ones.
Notably the biggest differences is in how the free space estimation
is computed. Before it was:

	free_estimated = (r_data_chunks - r_data_used) / data_ratio;

After it is:

	free_estimated = l_data_chunks - l_data_used;

which give the same results when there is no mixed raid level, but a
better result in the other case. I have to point out that before in the 
code there was a comment that said the opposite.

The other place where the r_{data,metadata}_used are use is for the
"Used:" field. For this case I estimated these values using the
following formula (only for raid5/6 profiles):

	r_data_used += (double)r_data_chunks * l_data_used /
                               l_data_chunks;

Note that this is not fully accurate. Eg. suppose to have two raid5 chunks,
the first one with 3 disks, the second one with 4 disks, and that each
chunk is 1GB.
r_data_chunks_r56, l_data_used_r56, l_data_chunks_r56 are completely defined,
but real r_data_used is completely different in these two cases:
- the first chunk is full and the second one id empty
- the first chunk is full empty and the second one is full
However now this error affect only the "Used:" field.


So now if you run btrfs fi us in a raid6 filesystem you get:

$ sudo btrfs fi us / 
Overall:
    Device size:		  40.00GiB
    Device allocated:		   8.28GiB
    Device unallocated:		  31.72GiB
    Device missing:		     0.00B
    Used:			   5.00GiB
    Free (estimated):		  17.36GiB	(min: 17.36GiB)
    Data ratio:			      2.00
    Metadata ratio:		      0.00
    Global reserve:		   3.25MiB	(used: 0.00B)

Data,RAID6: Size:4.00GiB, Used:2.50GiB (62.53%)
[...]

Instead before:

$ sudo btrfs fi us /
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
Overall:
    Device size:		  40.00GiB
    Device allocated:		     0.00B
    Device unallocated:		  40.00GiB
    Device missing:		     0.00B
    Used:			     0.00B
    Free (estimated):		     0.00B	(min: 8.00EiB)
    Data ratio:			      0.00
    Metadata ratio:		      0.00
    Global reserve:		   3.25MiB	(used: 0.00B)

Data,RAID6: Size:4.00GiB, Used:2.50GiB (62.53%)
[...]


I want to point out that this patch should be compatible with my
previous patches set (the ones related to the new ioctl 
BTRFS_IOC_GET_CHUNK_INFO). If both are merged we will have a 'btrfs fi us'
commands with full support a raid5/6 filesystem without needing root
capability.

Comments are welcome.
BR
G.Baroncelli

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