Soft Updates (mini) HOWTO

Soft Updates (mini) HOWTO

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	This HowTo will cover the installation of Soft Updates, including
how to do the necessary kernel modifications and enabling/disabling it for
the desired filesystems.


	1.	Background
	2.	Installation

	        2.1.    Installing Source Files
	        2.2.    Adding Kernel Option
	        2.2.    Adding Kernel Option
        	2.3.    Recompiling Kernel
		2.4.    Booting into Single User Mode and Unmounting Filesystems
        	2.5.    Enabling/Disabling Soft Updates

	3.	Appendix

	Soft Updates are a powerful method for increasing filesystem
performance. They decrease syncronous I/O and increase asyncronous I/O
which increases disk activity speed and decreases disk I/O through a
'trickle sync' facility that is incorporated into the kernel for more
efficient disk syncing operations. The code is very stable, although,
there have been some ocassional reports of difficulties. Soft Updates will
not cause filesystem corruption, but, if they begin causing difficulties,
then they can be turned off easily with the tunefs(8) command. To learn
more about how Soft Updates work, please read

	Before Soft Updates can be installed, the system must have the
kernel source so that the kernel can be recompiled with the necessary
kernel patches. To learn more about updating/downloading the
system/kernel source code, check out:  
and for building and installing a custom kernel, check out: This HowTo
will only in the most cursory fashion cover kernel compilation.

	2.	Installation

	If one is using an up-to-date (as of June 22, 2000) FreeBSD
4.0-STABLE, then one can skip to section 2.5 as Soft Updates have been
switched to the BSD license and merged into the base system.
	2.1.	Installing Source Files

	After the kernel source code is present in /usr/src/sys, one is
ready to begin the installation procedure for Soft Updates. The Soft
Updates files are located in /usr/src/sys/contrib/softupdates. They must
be linked into /usr/src/sys/ufs/ffs, which contains the source for the FFS

	cd /usr/src/sys/ufs/ffs
	ln -s ../../contrib/softupdates/*.[ch] .

	2.2.	Adding Kernel Option

	Next, the Soft Updates kernel option must be added into one's
kernel config. this can be easily done with your favourite text editor.
Add 'option SOFTUPDATES' and then save the kernel config.

	2.3.	Recompiling Kernel

	Now, one must recompile the kernel. Assuming that one's kernel
config is named SOFT, the following steps would suffice. Remember, you
must do this as root:

	cd /usr/src/sys/i386/conf
	/usr/sbin/config SOFT
	cd ../../compile/SOFT
	make depend; make; make install

	2.4.	Booting into Single User Mode and Unmounting Filesystems	

	Once the build and install is complete, one must reboot into
single user mode and umount every filesystem that one wishes to use with
Soft Updates. Remember, enabling Soft Updates like any other filesystem
tuning procedure must be done on unmounted filesystems; as such, executing
the procedure in single user mode is imperative. You can easily drop into
single user mode by typing:

	shutdown now

	Once in, type:

	umount -a

	If any filesystems are still in use and can not be umount(8)'ed,
then one will need to either stop the processes that are using
those filesystems or reboot. To reboot into single user mode, do the

	a) Type: reboot
	b) When the booter loads and begins the (default) 10 second
count-down prior to kernel loading, hit <space>. One will be dropped into
the booter prompt.
	c) Type: boot -s

	At this point, the kernel will load and the system will boot into
single user mode.

	2.5.	Enabling Soft Updates

	Now, one must enable the Soft Updates option for each
filesystem with tunefs(8). For instance, to enable Soft Updates for the
/usr filesystem, and /usr has a corresponding entry in /etc/fstab, the
following will accomplish this:

	tunefs -n enable /usr

	Enable Soft Updates for the filesystems with the highest I/O
activity. Filesystems such as /usr, /tmp, and /var are excellent
candidates. /home may also experience much I/O activity if one has many
users. / rarely experiences much I/O activity.

	Having enabled Soft Updates for the desired filesystems, type
'exit' at the single-user prompt, and the system will boot into multiuser
mode. Likewise, one can reboot and allow the system to boot into

	To verify that Soft Updates have been enabled, once in multiuser
mode, one can type 'mount' without any options and this will indicate the
'soft-updates' option for each filesystem for which Soft Updates had been
enabled. For instance:

(root@erudition)/usr/src/sys/ufs/ffs># mount
/dev/wd0s1a on / (ufs, NFS exported, local, writes: sync 92 async 54296)
/dev/wd0s1g on /home (ufs, NFS exported, local, soft-updates, writes: sync
29193 async 312766)
/dev/wd0s1e on /tmp (ufs, local, soft-updates, writes: sync 2 async 12961)
/dev/wd0s1h on /usr (ufs, NFS exported, local, soft-updates, writes: sync
4 async 266829)
/dev/wd0s1f on /var (ufs, local, soft-updates, writes: sync 549 async
procfs on /proc (procfs, local)
	Characteristic of Soft Updates performance, the 'sync' (syncronous
I/O) value will be drastically lower than the 'async' (asyncronous I/O)

	To disable Soft Updates for a particular filesystem, one must
likewise boot into single user mode and run 'tunefs -n disable' for the
intended filesystem.


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