[GIT PULL v4] Btrfs: improve write ahead log with sub transaction

[Liu Bo <liubo2009@cn.fujitsu.com>]

I've been working to try to improve the write-ahead log's performance,
and I found that the bottleneck addresses in the checksum items,
especially when we want to make a random write on a large file, e.g a 4G file.

Then a idea for this suggested by Chris is to use sub transaction ids and just
to log the part of inode that had changed since either the last log commit or
the last transaction commit.  And as we also push the sub transid into the btree
blocks, we'll get much faster tree walks.  As a result, we abandon the original
brute force approach, which is "to delete all items of the inode in log",
to making sure we get the most uptodate copies of everything, and instead
we manage to "find and merge", i.e. finding extents in the log tree and merging
in the new extents from the file.

This patchset puts the above idea into code, and although the code is now more
complex, it brings us a great deal of performance improvement:

in my sysbench "write + fsync" test:

        451.01Kb/sec -> 4.3621Mb/sec

Also, I've run the "synctest", and it works well with both directory and file.

v1->v2, rebase.
v2->v3, thanks to Chris, we worked together to solve 2 bugs, and after that it
worked as expected.
v3->v4, thanks to Josef, we simplify several codes.
 
Liu Bo (12):
  Btrfs: introduce sub transaction stuff
  Btrfs: update block generation if should_cow_block fails
  Btrfs: modify btrfs_drop_extents API
  Btrfs: introduce first sub trans
  Btrfs: still update inode trans stuff when size remains unchanged
  Btrfs: improve log with sub transaction
  Btrfs: add checksum check for log
  Btrfs: fix a bug of log check
  Btrfs: kick off useless code
  Btrfs: use the right generation number to read log_root_tree
  Btrfs: do not iput inode when inode is still in log
  Revert "Btrfs: do not flush csum items of unchanged file data during
    treelog"

 fs/btrfs/btrfs_inode.h |   12 ++-
 fs/btrfs/ctree.c       |   69 +++++++++++----
 fs/btrfs/ctree.h       |    5 +-
 fs/btrfs/disk-io.c     |   12 ++--
 fs/btrfs/extent-tree.c |   10 ++-
 fs/btrfs/file.c        |   22 ++---
 fs/btrfs/inode.c       |   39 ++++++---
 fs/btrfs/ioctl.c       |    6 +-
 fs/btrfs/relocation.c  |    6 +-
 fs/btrfs/transaction.c |   13 ++-
 fs/btrfs/transaction.h |   19 ++++-
 fs/btrfs/tree-defrag.c |    2 +-
 fs/btrfs/tree-log.c    |  225 ++++++++++++++++++++++++++++++++----------------
 13 files changed, 293 insertions(+), 147 deletions(-)