$FreeBSD$
Using Soft Updates
To enable the soft updates feature in your kernel, add option
SOFTUPDATES to your kernel configuration.
Once you are running a kernel with soft update support, you need to enable
it for whichever filesystems you wish to run with the soft update policy.
This is done with the -n option to tunefs(8) on the UNMOUNTED filesystems,
e.g. from single-user mode you'd do something like:
tunefs -n enable /usr
To permanently enable soft updates on the /usr filesystem (or at least
until a corresponding ``tunefs -n disable'' is done).
Soft Updates Copyright Restrictions
As of June 2000 the restrictive copyright has been removed and
replaced with a `Berkeley-style' copyright. The files implementing
soft updates now reside in the sys/ufs/ffs directory and are
compiled into the generic kernel by default.
Soft Updates Status
The soft updates code has been running in production on many
systems for the past two years generally quite successfully.
The two current sets of shortcomings are:
1) On filesystems that are chronically full, the two minute lag
from the time a file is deleted until its free space shows up
will result in premature filesystem full failures. This
failure mode is most evident in small filesystems such as
the root. For this reason, use of soft updates is not
recommended on the root filesystem.
2) If your system routines runs parallel processes each of which
remove many files, the kernel memory rate limiting code may
not be able to slow removal operations to a level sustainable
by the disk subsystem. The result is that the kernel runs out
of memory and hangs.
Both of these problems are being addressed, but have not yet
been resolved. There are no other known problems at this time.
How Soft Updates Work
For more general information on soft updates, please see:
http://www.mckusick.com/softdep/
http://www.ece.cmu.edu/~ganger/papers/CSE-TR-254-95/
--
Marshall Kirk McKusick <mckusick@mckusick.com>
July 2000
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