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NetBSD 6.1.5 - man page for wapbl (netbsd section 4)

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

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

SYNOPSIS
     options WAPBL
     options WAPBL_DEBUG

DESCRIPTION
     The WAPBL driver provides meta-data journaling for file systems.  In particular, it is used
     with the fast file system (FFS) to provide rapid file system consistency checking after a
     system outage.  It also provides better 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 file system and the end of the partition,
     then unless the journal size has been specified with tunefs(8) then the journal will be cre-
     ated after the file system.  To obtain space between the file system and the end of the par-
     tition 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 exam-
     ple, a swap partition can be shrunk in order to accommodate the journal after the file sys-
     tem on a partition before the swap partition.

     For a new file system,

	   newfs -s -64m wd0a

     can be used to leave space for a 64MB journal at the end of /dev/wd0a.

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

	   tunefs -l 64m wd0a

     to indicate that a journal of size 64MB on the file system on /dev/wd0a 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/wd0a /mnt

     will be sufficient to create an appropriate journal within the file system.  Running

	   tunefs -l 0 wd0a

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

	   tunefs -l 0 wd0a

     followed by

	   mount -o log /dev/wd0a /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.

     For kernel developers, the compile time option WAPBL_DEBUG turns on debugging.

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

HISTORY
     WAPBL was originally written by Darrin B. Jewell while at Wasabi Systems Inc.  Wasabi Sys-
     tems 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.

CAVEATS
     Older releases of the system, and other systems that support the UFS format should only
     access WAPBL file systems in read-only mode.  Additionally, the fsck(8) command from such
     systems should not be run against WAPBL file systems.  Failure to observe these guidelines
     may damage the file system.

     WAPBL requires the super block to be in the UFS2 format.  The super block format can be
     checked using the -s option with dumpfs(8), and 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 all outstanding metadata transactions to be committed to disk, introducing
     additional latency.  This can have an impact on database software and other software that
     calls fsync(2) often.

     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.

BSD					  July 26, 2012 				      BSD


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