Re: [PATCH] btrfs: allow degenerate raid0/raid10

[David Sterba <dsterba@suse.cz>]

On Fri, Jul 23, 2021 at 10:27:30PM +0500, Roman Mamedov wrote:
> On Fri, 23 Jul 2021 16:08:43 +0200
> David Sterba <dsterba@suse.cz> wrote:
> 
> > > Can we slightly enhance the output?
> > > RAID0/1 really looks like a new profile now, even the "1" really means
> > > the number of device.
> > 
> > Do you have a concrete suggestion? This format was inspired by a
> > discussion and suggested by users so I guess this is what people expect
> > and I find it clear. It's also documented in manual page so if you think
> > it's not clear or missing some important information, please let me
> > know.
> 
> It really reads like another RAID level, easily confused with RAID10.
> 
> Or that it would flip between RAID0 and RAID1 depending on something.

I think it could be confusing when the number of stripes is also another
raid level, like /1 in this case. From the commit
https://github.com/kdave/btrfs-progs/commit/4693e8226140289dcf8f0932af05895a38152817

/dev/vdc, ID: 1
   Device size:             1.00GiB
   Device slack:              0.00B
   Data,RAID0/2:          912.62MiB
   Data,RAID0/3:          912.62MiB
   Metadata,RAID1:        102.38MiB
   System,RAID1:            8.00MiB
   Unallocated:             1.00MiB

it's IMHO clear or at least prompting to read the docs what it means.

> Maybe something like RAID0d1?

That looks similar to RAID1c3 which I'd interpret as a new profile as
well. The raid56 profiles also print the stripe count so I don't know if
eg. RAID5d4 is really an improvement.

A 4 device mix of raid56 data and metadata would look like:

# btrfs dev us .
/dev/sda10, ID: 1
   Device size:            10.00GiB
   Device slack:              0.00B
   Data,RAID5/4:            1.00GiB
   Metadata,RAID6/4:       64.00MiB
   System,RAID6/4:          8.00MiB
   Unallocated:             8.93GiB

/dev/sda11, ID: 2
   Device size:            10.00GiB
   Device slack:              0.00B
   Data,RAID5/4:            1.00GiB
   Metadata,RAID6/4:       64.00MiB
   System,RAID6/4:          8.00MiB
   Unallocated:             8.93GiB

/dev/sda12, ID: 3
   Device size:            10.00GiB
   Device slack:              0.00B
   Data,RAID5/4:            1.00GiB
   Metadata,RAID6/4:       64.00MiB
   System,RAID6/4:          8.00MiB
   Unallocated:             8.93GiB

/dev/sda13, ID: 4
   Device size:            10.00GiB
   Device slack:              0.00B
   Data,RAID5/4:            1.00GiB
   Metadata,RAID6/4:       64.00MiB
   System,RAID6/4:          8.00MiB
   Unallocated:             8.93GiB

Maybe it's still too new so nobody is used to it and we've always had
problems with the raid naming scheme anyway.