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WAPBL(4)                 BSD Kernel Interfaces Manual                 WAPBL(4)

NAME
     WAPBL -- Write Ahead Physical Block Logging file system journaling

SYNOPSIS
     options WAPBL
     options WAPBL_DEBUG
     options WAPBL_DEBUG_SERIALIZE

DESCRIPTION
     The WAPBL driver provides meta-data journaling for file systems.  In par-
     ticular, it is used with the fast file system (FFS) to provide rapid file
     system consistency checking after a system outage.  It also provides bet-
     ter general-use performance over regular FFS.

     WAPBL currently maintains its journal in one of two locations:

     - After the file system
             The journal is placed in the same partition as the file system,
             but between the file system and the end of the partition.

     - Within the file system
             The journal is allocated as a special contiguous file within the
             file system.  The journal file is not visible via normal file
             system access.

     A new journal is created automatically when a file system is mounted via
     mount(8) with the -o log option.  If no journal size has been specified
     with tunefs(8), then the size of the journal will be based on 1MB of
     journal per 1GB of file system, to a maximum journal size of 64MB.

     If there is adequate space between the end of the the file system and the
     end of the partition, then unless the journal size has been specified
     with tunefs(8) then the journal will be created after the file system.
     To obtain space between the file system and the end of the partition the
     size of the partition can be adjusted using disklabel(8).  Care must be
     taken not to damage existing data on existing partitions, but this method
     will work well if, for example, a swap partition can be shrunk in order
     to accommodate the journal after the file system on a partition before
     the swap partition.

     For a new file system,

           newfs -s -64m /dev/rwd0a

     can be used to leave space for a 64MB journal at the end of /dev/rwd0a.

     To specify the size of the journal within the file system tunefs(8) can
     be used as follows:

           tunefs -l 64m /dev/rwd0a

     to indicate that a journal of size 64MB on the file system on /dev/rwd0a
     should be created the next time that file system is mounted.  This must
     be done before the file system is mounted with the ``-o log'' option.
     For existing file systems and general use, however, simply using

           mount -o log /dev/rwd0a /mnt

     will be sufficient to create an appropriate journal within the file sys-
     tem.  Running

           tunefs -l 0 /dev/rwd0a

     will schedule the log for removal on the next read-write mount, and run-
     ning

           tunefs -l 0 /dev/rwd0a

     followed by

           mount -o log /dev/rwd0a /mnt

     will remove the log and then re-create it with the default size.  This
     method can also be used to grow or shrink the size of the journal.

     With the journal, fsck(8) is no longer required at system boot.  If the
     system has been shutdown in an unclean fashion then the journal will be
     replayed when the file system is mounted.  fsck(8) can still be used to
     force a consistency check of the file system should that be desired.

     WAPBL(8) has a number of debugging options.  The option

           options WAPBL_DEBUG

     turns on general debugging.  The option

           options WAPBL_DEBUG_SERIALIZE

     forces the serialization of all IO.  This is currently be used to help
     alleviate a performance issue seen on multi-core machines, where multiple
     simultaneous extractions of tar-files can cause degenerate performance.

SEE ALSO
     config(1), fsck(8), newfs(8), mount(8), umount(8)

CAVEATS
     An unreplayed WAPBL journal (eg after a crash or power failure) may cause
     problems if the file system is then used with an older kernel or userland
     what isn't WAPBL aware.

     An older fsck(8) that isn't WAPBL aware will not be able to deal with an
     in-filesystem log.

     WAPBL requires the super block to be in the UFS2 format.  Older FFSv1
     file systems will need to be updated to the newer super block layout with
     the -c option to fsck_ffs(8).

     fsync(2) causes the journal to be committed to disk, resulting in non-
     negligible performance issues.  This can have an impact on database soft-
     ware and other software that calls fsync(2) often.

     File system snapshots (fss(4)) currently do not work with WAPBL.

     There are some pending issues with high metadata-rate workloads (like
     multiple simultaneous tar-file extractions) resulting in degenerate file
     system performance.  The current work-around is to use

           options WAPBL_DEBUG_SERIALIZE

     as mentioned above.

     In-file system log allocation should be done on a relatively quiet file
     system.  The error path for log allocation failures could result in a
     ``dangling inode'' issue, requiring an fsck(8) to fix.

     If the file system is full or close to full then making a journal can
     fail.

HISTORY
     WAPBL was originally written by Darrin B. Jewell while at Wasabi Systems
     Inc.  Wasabi Systems contributed the code to NetBSD and was integrated by
     Simon Burge, Antti Kantee, Andy Doran, and Greg Oster.

     WAPBL first appeared in NetBSD 5.0.

BSD                              July 21, 2008                             BSD