3- and 4- copy RAID1

Hi,

I have some goodies that go into the RAID56 problem, although not implementing all the remaining features, it can be useful independently.

This time my hackweek project

https://hackweek.suse.com/17/projects/do-something-about-btrfs-and-raid56

aimed to implement the fix for the write hole problem but I spent more time with analysis and design of the solution and don't have a working prototype for that yet.

This patchset brings a feature that will be used by the raid56 log, the log has to be on the same redundancy level and thus we need a 3-copy replication for raid6. As it was easy to extend to higher replication, I've added a 4-copy replication, that would allow triple copy raid (that does not have a standardized name).

The number of copies is fixed, so it's not N-copy for an arbitrary N. This would complicate the implementation too much, though I'd be willing to add a 5-copy replication for a small bribe.

The new raid profiles and covered by an incompatibility bit, called extended_raid, the (idealistic) plan is to stuff as many new raid-related features as possible. The patch 4/4 mentions the 3- 4- copy raid1, configurable stripe length, write hole log and triple parity. If the plan turns out to be too ambitious, the ready and implemented features will be split and merged.

An interesting question is the naming of the extended profiles. I picked something that can be easily understood but it's not a final proposal. Years ago, Hugo proposed a naming scheme that described the non-standard raid varieties of the btrfs flavor:

https://marc.info/?l=linux-btrfs&m=136286324417767

Switching to this naming would be a good addition to the extended raid.

Regarding the missing raid56 features, I'll continue working on them as time permits in the following weeks/months, as I'm not aware of anybody working on that actively enough so to speak.

Anyway, git branches with the patches:

kernel: git://github.com/kdave/btrfs-devel dev/extended-raid-ncopies
progs:  git://github.com/kdave/btrfs-progs dev/extended-raid-ncopies

David Sterba (4):
btrfs: refactor block group replication factor calculation to a helper
btrfs: add support for 3-copy replication (raid1c3)
btrfs: add support for 4-copy replication (raid1c4)
btrfs: add incompatibility bit for extended raid features

 fs/btrfs/ctree.h                |  1 +
 fs/btrfs/extent-tree.c          | 45 +++++++-----------
 fs/btrfs/relocation.c           |  1 +
 fs/btrfs/scrub.c                |  4 +-
 fs/btrfs/super.c                | 17 +++----
 fs/btrfs/sysfs.c                |  2 +
 fs/btrfs/volumes.c              | 84 ++++++++++++++++++++++++++++++---
 fs/btrfs/volumes.h              |  6 +++
 include/uapi/linux/btrfs.h      | 12 ++++-
 include/uapi/linux/btrfs_tree.h |  6 +++
 10 files changed, 134 insertions(+), 44 deletions(-)

2.18.0