Re: [RFC PATCH 2/3] btrfs-progs: kernel based default features for mkfs

[Anand Jain <anand.jain@oracle.com>]

Hi,

  more details about this RFC as below..


> So a user with old kernel won't be able to create a filesystem with
> newer feature forever.

  Thats not true. Here below is an example of the problem and fix
  in action.

> Maybe the user are just making btrfs for his or her newer kernel?

  Yes. Thats still possible as below.



[[current problem]]

I am on 3.8 kernel which does not support the skinny-metadata feature.
---------
# uname -r
3.8.13-98.4.1.el6uek.x86_64
-------------

Lets say customer upgraded the latest btrfs-progs as they wanted better
cli interface, reporting and document which is only available in the
latest btrfs-progs.

As btrfs-progs is backward kernel compatible, they don't have to
upgrade the kernel.

But as skinny-metadata is part of the "mkfs default feature", thats get
enabled by default during mkfs.

----------
# mkfs.btrfs -f /dev/sdc
btrfs-progs v4.2.2-7-g03cf344
See http://btrfs.wiki.kernel.org for more information.

Label:              (null)
UUID:               bb85b156-35eb-49a5-be5a-512fec1abab4
Node size:          16384
Sector size:        4096
Filesystem size:    3.00GiB
Block group profiles:
   Data:             single            8.00MiB
   Metadata:         DUP             161.56MiB
   System:           DUP              12.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata         <-----
Number of devices:  1
Devices:
    ID        SIZE  PATH
     1     3.00GiB  /dev/sdc
-------------------

mount on the running kernel 3.8 fails, as there is no kernel support
for skinny-metadata
-----------
# mount /dev/sdc /btrfs
mount: wrong fs type, bad option, bad superblock on /dev/sdc,
        missing codepage or helper program, or other error
        In some cases useful info is found in syslog - try
        dmesg | tail  or so

BTRFS: couldn't mount because of unsupported optional features (100).
btrfs: open_ctree failed
----------

Customers need to upgrade the kernel also to mount this FS.


[[current solution]]

User must know that their running kernel does not support
skinny metadata and disable features accordingly as below.

------------
# mkfs.btrfs -f -O ^skinny-metadata /dev/sdc > /dev/null && mount 
/dev/sdc /btrfs
# echo $?
0
------------


[[problem with the current solution]]

btrfs-progs is backward kernel compatible.

But the default feature as set during mkfs is very btrfs-progs centric,
and is not in sync with the current running kernel.


[[new proposed, with the patch]]

- the default-features that are not supported by the running kernel
  are masked.

  So now the default mkfs.btrfs and mount works. As progs finds that
  kernel version is too old to support skinny-metadata and remotes is
  from the default feature list.

-------------------
# uname -r
3.8.13-98.4.1.el6uek.x86_64


# mkfs.btrfs -f /dev/sdc
btrfs-progs v4.2.2-7-g03cf344
See http://btrfs.wiki.kernel.org for more information.

Label:              (null)
UUID:               f2513ff0-6d94-4b6a-8bbf-ff53d343fa62
Node size:          16384
Sector size:        4096
Filesystem size:    3.00GiB
Block group profiles:
   Data:             single            8.00MiB
   Metadata:         DUP             161.56MiB
   System:           DUP              12.00MiB
SSD detected:       no
Incompat features:  extref  <-----
Number of devices:  1
Devices:
    ID        SIZE  PATH
     1     3.00GiB  /dev/sdc

# mount /dev/sdc /btrfs
-------------------


However as Qu pointed out if user wants to create FS with a feature that
is not supported by running kernel. They still have the choice to enable
it using -O option. as below. however they won't be able to mount it
unless kernel is upgraded as well.

-------
# mkfs.btrfs -f -O skinny-metadata /dev/sdc
btrfs-progs v4.2.2-7-g03cf344
See http://btrfs.wiki.kernel.org for more information.

Label:              (null)
UUID:               30c018ab-3d2a-4acc-8287-3f28c5324fca
Node size:          16384
Sector size:        4096
Filesystem size:    3.00GiB
Block group profiles:
   Data:             single            8.00MiB
   Metadata:         DUP             161.56MiB
   System:           DUP              12.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata   <----
Number of devices:  1
Devices:
    ID        SIZE  PATH
     1     3.00GiB  /dev/sdc

# mount /dev/sdc /btrfs
mount: wrong fs type, bad option, bad superblock on /dev/sdc,
        missing codepage or helper program, or other error
        In some cases useful info is found in syslog - try
        dmesg | tail  or so
#
---------

In nut shell this patch makes _mkfs and btrfs-convert default features_
dynamically aligned with the running kernel.

Thanks, Anand