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+.\"     $NetBSD: wapbl.4,v 1.16 2013/05/29 14:55:24 wiz Exp $
+.\"
+.\" Copyright (c) 2008, 2009, 2012 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd December 3, 2012
+.Dt WAPBL 4
+.Os
+.Sh NAME
+.Nm WAPBL
+.Nd Write Ahead Physical Block Logging file system journaling
+.Sh SYNOPSIS
+.Cd options WAPBL
+.Cd options WAPBL_DEBUG
+.Sh DESCRIPTION
+The
+.Nm
+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.
+.Pp
+WAPBL currently maintains its journal in one of two locations:
+.Bl -tag -width indent
+.It - 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.
+.It - 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.
+.El
+.Pp
+A new journal is created automatically when a file system is mounted
+via
+.Xr mount 8
+with the
+.Fl o Ar log
+option.
+If no journal size has been specified with
+.Xr 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.
+.Pp
+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
+.Xr 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
+.Xr 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.
+.Pp
+For a new file system,
+.Bd -literal -offset indent
+newfs -s -64m wd0a
+.Ed
+.Pp
+can be used to leave space for a 64MB journal at the end of
+.Pa /dev/wd0a .
+.Pp
+To specify the size of the journal within the file system
+.Xr tunefs 8
+can be used as follows:
+.Bd -literal -offset indent
+tunefs -l 64m wd0a
+.Ed
+.Pp
+to indicate that a journal of size 64MB on the file system on
+.Pa /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
+.Dq -o log
+option.
+For existing file systems and general use, however, simply using
+.Bd -literal -offset indent
+mount -o log /dev/wd0a /mnt
+.Ed
+.Pp
+will be sufficient to create an appropriate journal within the file
+system.
+Running
+.Bd -literal -offset indent
+tunefs -l 0 wd0a
+.Ed
+.Pp
+will schedule the log for removal on the next read-write mount, and
+running
+.Bd -literal -offset indent
+tunefs -l 0 wd0a
+.Ed
+.Pp
+followed by
+.Bd -literal -offset indent
+mount -o log /dev/wd0a /mnt
+.Ed
+.Pp
+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
+by first scheduling the log for removal, then mounting read-write, but
+with logging disabled (so no new log will be created), then unmounting
+again, setting the desired log size and finally re-mounting with logging
+enabled.
+.Pp
+With the journal,
+.Xr 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.
+.Xr fsck 8
+can still be used to force a consistency check of the file system
+should that be desired.
+.Pp
+For kernel developers, the compile time option
+.Dv WAPBL_DEBUG
+turns on debugging.
+.Sh SEE ALSO
+.Xr config 1 ,
+.Xr fsck 8 ,
+.Xr mount 8 ,
+.Xr newfs 8 ,
+.Xr umount 8
+.Sh HISTORY
+.An -nosplit
+.Nm
+was originally written by
+.An Darrin B. Jewell
+while at Wasabi Systems Inc.
+Wasabi Systems contributed the code to
+.Nx ,
+and it was integrated by
+.An Simon Burge ,
+.An Antti Kantee ,
+.An Andy Doran ,
+and
+.An Greg Oster .
+.Pp
+.Nm
+first appeared in
+.Nx 5.0 .
+.Sh CAVEATS
+Older releases of the system, and other systems that support the
+.Dv UFS
+format should only access
+.Nm
+file systems in read-only mode.
+Additionally, the
+.Xr fsck 8
+command from such systems should not be run against
+.Nm
+file systems.
+Failure to observe these guidelines may damage the file system.
+.Pp
+.Nm
+requires the super block to be in the UFS2 format.
+The super block format can be checked using the
+.Fl s
+option with
+.Xr dumpfs 8 ,
+and older FFSv1 file systems will need to be updated to the newer
+super block layout with the
+.Fl c
+option to
+.Xr fsck_ffs 8 .
+.Pp
+.Xr 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
+.Xr fsync 2
+often.
+.Pp
+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
+.Dq dangling inode
+issue, requiring an
+.Xr fsck 8
+to fix.