git: HAMMER VFS - Change default from version 3 to version 4

Matthew Dillon dillon at
Sat Dec 5 09:28:34 PST 2009

commit fd74884e5f97b1cbfca7c582336d7fbbbdf07644
Author: Matthew Dillon <dillon at>
Date:   Sat Dec 5 09:20:27 2009 -0800

    HAMMER VFS - Change default from version 3 to version 4
    * Version 4 is now the default for newfs_hammer and is no longer
      considered a work in progress.
    * Filesystems may be upgraded to version 4 in-place.  The UNDO FIFO
      (typically 1G) is reformatted so the upgrade might take a minute or
      two depending.
    * Version 4 allows the UNDO FIFO to be flushed without also having
      to flush the volume header, removing 2 of the 4 disk syncs typically
      required for an fsync() and removing 1 of the 2 disk syncs typically
      required for a flush sequence.
      This is accomplished by adding a sequence number of the UNDO entries
      in the FIFO and preventing them from crossing a 512-byte boundary,
      so each 512-byte block starts with an entry.  HAMMER's mount code
      is then able to scan the UNDO FIFO to determine the start and end
      for recovery purposes.
    * Version 4 implements the infrastructure needed for REDO, but does not
      implement any REDO operations.

Summary of changes:
 sys/vfs/hammer/hammer_disk.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

DragonFly BSD source repository

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