Hi Liu,

On 2017-08-01 18:14, Liu Bo wrote:

This aims to fix write hole issue on btrfs raid5/6 setup by adding a
separate disk as a journal (aka raid5/6 log), so that after unclean
shutdown we can make sure data and parity are consistent on the raid
array by replaying the journal.

it would be possible to have more information ?
- what is logged ? data, parity or data + parity ?
- in the past I thought that it would be sufficient to log only the stripe position involved by a RMW cycle, and then start a scrub on these stripes in case of an unclean shutdown: do you think that it is feasible ?
- does this journal disk also host other btrfs log ?

The idea and the code are similar to the write-through mode of md
raid5-cache, so ppl(partial parity log) is also feasible to implement.
(If you've been familiar with md, you may find this patch set is
boring to read...)

Patch 1-3 are about adding a log disk, patch 5-8 are the main part of
the implementation, the rest patches are improvements and bugfixes,
eg. readahead for recovery, checksum.

Two btrfs-progs patches are required to play with this patch set, one
is to enhance 'btrfs device add' to add a disk as raid5/6 log with the
option '-L', the other is to teach 'btrfs-show-super' to show
%journal_tail.

This is currently based on 4.12-rc3.

The patch set is tagged with RFC, and comments are always welcome,
thanks.

Known limitations:
- Deleting a log device is not implemented yet.


Liu Bo (14):
  Btrfs: raid56: add raid56 log via add_dev v2 ioctl
  Btrfs: raid56: do not allocate chunk on raid56 log
  Btrfs: raid56: detect raid56 log on mount
  Btrfs: raid56: add verbose debug
  Btrfs: raid56: add stripe log for raid5/6
  Btrfs: raid56: add reclaim support
  Btrfs: raid56: load r5log
  Btrfs: raid56: log recovery
  Btrfs: raid56: add readahead for recovery
  Btrfs: raid56: use the readahead helper to get page
  Btrfs: raid56: add csum support
  Btrfs: raid56: fix error handling while adding a log device
  Btrfs: raid56: initialize raid5/6 log after adding it
  Btrfs: raid56: maintain IO order on raid5/6 log

 fs/btrfs/ctree.h                |   16 +-
 fs/btrfs/disk-io.c              |   16 +
 fs/btrfs/ioctl.c                |   48 +-
 fs/btrfs/raid56.c               | 1429 ++++++++++++++++++++++++++++++++++-----
 fs/btrfs/raid56.h               |   82 +++
 fs/btrfs/transaction.c          |    2 +
 fs/btrfs/volumes.c              |   56 +-
 fs/btrfs/volumes.h              |    7 +-
 include/uapi/linux/btrfs.h      |    3 +
 include/uapi/linux/btrfs_tree.h |    4 +
 10 files changed, 1487 insertions(+), 176 deletions(-)

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