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WAPBL(4) NetBSD Kernel Interfaces Manual WAPBL(4)
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by Kimmo Suominen.
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WAPBL -- Write Ahead Physical Block Logging file system journaling
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
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 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 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 sys-
tunefs -l 0 wd0a
will schedule the log for removal on the next read-write mount, and run-
tunefs -l 0 wd0a
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 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
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
config(1), fsck(8), mount(8), newfs(8), umount(8)
WAPBL was originally written by Darrin B. Jewell while at Wasabi Systems
Inc. Wasabi Systems contributed the code to NetBSD, and it was inte-
grated by Simon Burge, Antti Kantee, Andy Doran, and Greg Oster.
WAPBL first appeared in NetBSD 5.0.
Older releases of the system, and other systems that support the UFS for-
mat should only access WAPBL file systems in read-only mode. Addition-
ally, the fsck(8) command from such systems should not be run against
WAPBL file systems. Failure to observe these guidelines may damage the
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 data-
base 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.
NetBSD 9.1 December 3, 2012 NetBSD 9.1