25.7 Rebuilding “world”

Once you have synchronized your local source tree against a particular version of FreeBSD (FreeBSD-STABLE, FreeBSD-CURRENT, and so on) you can then use the source tree to rebuild the system.

Make a Backup: It cannot be stressed enough how important it is to make a backup of your system before you do this. While rebuilding the world is (as long as you follow these instructions) an easy task to do, there will inevitably be times when you make mistakes, or when mistakes made by others in the source tree render your system unbootable.

Make sure you have taken a backup. And have a fixit floppy or bootable CD at hand. You will probably never have to use it, but it is better to be safe than sorry!

Subscribe to the Right Mailing List: The FreeBSD-STABLE and FreeBSD-CURRENT branches are, by their nature, in development. People that contribute to FreeBSD are human, and mistakes occasionally happen.

Sometimes these mistakes can be quite harmless, just causing your system to print a new diagnostic warning. Or the change may be catastrophic, and render your system unbootable or destroy your file systems (or worse).

If problems like these occur, a “heads up” is posted to the appropriate mailing list, explaining the nature of the problem and which systems it affects. And an “all clear” announcement is posted when the problem has been solved.

If you try to track FreeBSD-STABLE or FreeBSD-CURRENT and do not read the FreeBSD-STABLE mailing list or the FreeBSD-CURRENT mailing list respectively, then you are asking for trouble.

Do Not Use make world: A lot of older documentation recommends using make world for this. Doing that skips some important steps and should only be used if you are sure of what you are doing. For almost all circumstances make world is the wrong thing to do, and the procedure described here should be used instead.

25.7.1 The Canonical Way to Update Your System

To update your system, you should check /usr/src/UPDATING for any pre-buildworld steps necessary for your version of the sources and then use the procedure outlined here.

These upgrade steps assume that you are currently using an old FreeBSD version, consisting of an old compiler, old kernel, old world and old configuration files. By “world” here we mean the core system binaries, libraries and programming files. The compiler is part of “world”, but has a few special concerns.

We also assume that you have already obtained the sources to a newer system. If the sources available on the particular system are old too, see Section 25.6 for detailed help about synchronizing them to a newer version.

Updating the system from sources is a bit more subtle than it might initially seem to be, and the FreeBSD developers have found it necessary over the years to change the recommended approach fairly dramatically as new kinds of unavoidable dependencies come to light. The rest of this section describes the rationale behind the currently recommended upgrade sequence.

Any successful update sequence must deal with the following issues:

These first two issues are the basis for the core buildworld, buildkernel, installkernel, installworld sequence that we describe in the following paragraphs. This is not an exhaustive list of all the reasons why you should prefer the currently recommended upgrade process. Some of the less obvious ones are listed below:

These concerns have led to the following recommended sequence. Note that the detailed sequence for particular updates may require additional steps, but this core process should remain unchanged for some time:

  1. make buildworld

    This first compiles the new compiler and a few related tools, then uses the new compiler to compile the rest of the new world. The result ends up in /usr/obj.

  2. make buildkernel

    Unlike the older approach, using config(8) and make(1), this uses the new compiler residing in /usr/obj. This protects you against compiler-kernel mismatches.

  3. make installkernel

    Place the new kernel and kernel modules onto the disk, making it possible to boot with the newly updated kernel.

  4. Reboot into single user mode.

    Single user mode minimizes problems from updating software that is already running. It also minimizes any problems from running the old world on a new kernel.

  5. mergemaster -p

    This does some initial configuration file updates in preparation for the new world. For instance it may add new user groups to the system, or new user names to the password database. This is often necessary when new groups or special system-user accounts have been added since the last update, so that the installworld step will be able to use the newly installed system user or system group names without problems.

  6. make installworld

    Copies the world from /usr/obj. You now have a new kernel and new world on disk.

  7. mergemaster

    Now you can update the remaining configuration files, since you have a new world on disk.

  8. Reboot.

    A full machine reboot is needed now to load the new kernel and new world with new configuration files.

Note that if you are upgrading from one release of the same FreeBSD branch to a more recent release of the same branch, i.e., from 7.0 to 7.1, then this procedure may not be absolutely necessary, since you are unlikely to run into serious mismatches between compiler, kernel, userland and configuration files. The older approach of make world followed by building and installing a new kernel might work well enough for minor updates.

But, when upgrading across major releases, people who do not follow this procedure should expect some problems.

It is also worth noting that many upgrades (i.e., 4.X to 5.0) may require specific additional steps (renaming or deleting specific files prior to installworld, for instance). Read the /usr/src/UPDATING file carefully, especially at the end, where the currently recommended upgrade sequence is explicitly spelled out.

This procedure has evolved over time as the developers have found it impossible to completely prevent certain kinds of mismatch problems. Hopefully, the current procedure will remain stable for a long time.

To summarize, the currently recommended way of upgrading FreeBSD from sources is:

# cd /usr/src
# make buildworld
# make buildkernel
# make installkernel
# shutdown -r now

Note: There are a few rare cases when an extra run of mergemaster -p is needed before the buildworld step. These are described in UPDATING. In general, though, you can safely omit this step if you are not updating across one or more major FreeBSD versions.

After installkernel finishes successfully, you should boot in single user mode (i.e., using boot -s from the loader prompt). Then run:

# mount -u /
# mount -a -t ufs
# adjkerntz -i
# mergemaster -p
# cd /usr/src
# make installworld
# mergemaster
# reboot

Read Further Explanations: The sequence described above is only a short resume to help you getting started. You should however read the following sections to clearly understand each step, especially if you want to use a custom kernel configuration.

25.7.2 Read /usr/src/UPDATING

Before you do anything else, read /usr/src/UPDATING (or the equivalent file wherever you have a copy of the source code). This file should contain important information about problems you might encounter, or specify the order in which you might have to run certain commands. If UPDATING contradicts something you read here, UPDATING takes precedence.

Important: Reading UPDATING is not an acceptable substitute for subscribing to the correct mailing list, as described previously. The two requirements are complementary, not exclusive.

25.7.3 Check /etc/make.conf

make(1) options are shown in make.conf(5) and /usr/share/examples/etc/make.conf. These settings can be added to /etc/make.conf to control the way make(1) runs and how it builds programs. Changes to some settings can have far-reaching and potentially surprising effects. Read the comments in both locations and keep in mind that the defaults have been chosen for a combination of performance and safety.

Options set in /etc/make.conf take effect every time make(1) is used, including compiling applications from the Ports Collection or user-written C programs, or building the FreeBSD operating system itself.

25.7.4 Check /etc/src.conf

/etc/src.conf controls the building of the operating system from source code. Unlike /etc/make.conf, the contents of /etc/src.conf only take effect when the FreeBSD operating system itself is being built. Descriptions of the many options available for this file are shown in src.conf(5). Be cautious about disabling seemingly unneeded kernel modules and build options. Sometimes there are unexpected or subtle interactions.

25.7.5 Update the Files in /etc

The /etc directory contains a large part of your system's configuration information, as well as scripts that are run at system startup. Some of these scripts change from version to version of FreeBSD.

Some of the configuration files are also used in the day to day running of the system. In particular, /etc/group.

There have been occasions when the installation part of make installworld has expected certain usernames or groups to exist. When performing an upgrade it is likely that these users or groups did not exist. This caused problems when upgrading. In some cases make buildworld will check to see if these users or groups exist.

An example of this is when the smmsp user was added. Users had the installation process fail for them when mtree(8) was trying to create /var/spool/clientmqueue.

The solution is to run mergemaster(8) in pre-buildworld mode by providing the -p option. This will compare only those files that are essential for the success of buildworld or installworld.

Tip: If you are feeling particularly paranoid, you can check your system to see which files are owned by the group you are renaming or deleting:

# find / -group GID -print

will show all files owned by group GID (which can be either a group name or a numeric group ID).

25.7.6 Drop to Single User Mode

You may want to compile the system in single user mode. Apart from the obvious benefit of making things go slightly faster, reinstalling the system will touch a lot of important system files, all the standard system binaries, libraries, include files and so on. Changing these on a running system (particularly if you have active users on the system at the time) is asking for trouble.

Another method is to compile the system in multi-user mode, and then drop into single user mode for the installation. If you would like to do it this way, simply hold off on the following steps until the build has completed. You can postpone dropping to single user mode until you have to installkernel or installworld.

As the superuser, you can execute:

# shutdown now

from a running system, which will drop it to single user mode.

Alternatively, reboot the system, and at the boot prompt, select the “single user” option. The system will then boot single user. At the shell prompt you should then run:

# fsck -p
# mount -u /
# mount -a -t ufs
# swapon -a

This checks the file systems, remounts / read/write, mounts all the other UFS file systems referenced in /etc/fstab and then turns swapping on.

Note: If your CMOS clock is set to local time and not to GMT (this is true if the output of the date(1) command does not show the correct time and zone), you may also need to run the following command:

# adjkerntz -i

This will make sure that your local time-zone settings get set up correctly — without this, you may later run into some problems.

25.7.7 Remove /usr/obj

As parts of the system are rebuilt they are placed in directories which (by default) go under /usr/obj. The directories shadow those under /usr/src.

You can speed up the make buildworld process, and possibly save yourself some dependency headaches by removing this directory as well.

Some files below /usr/obj may have the immutable flag set (see chflags(1) for more information) which must be removed first.

# cd /usr/obj
# chflags -R noschg *
# rm -rf *

25.7.8 Recompile the Base System Saving the Output

It is a good idea to save the output you get from running make(1) to another file. If something goes wrong you will have a copy of the error message. While this might not help you in diagnosing what has gone wrong, it can help others if you post your problem to one of the FreeBSD mailing lists.

The easiest way to do this is to use the script(1) command, with a parameter that specifies the name of the file to save all output to. You would do this immediately before rebuilding the world, and then type exit when the process has finished.

# script /var/tmp/mw.out
Script started, output file is /var/tmp/mw.out
# make TARGET
… compile, compile, compile …
# exit
Script done, …

If you do this, do not save the output in /tmp. This directory may be cleared next time you reboot. A better place to store it is in /var/tmp (as in the previous example) or in root's home directory. Compile the Base System

You must be in the /usr/src directory:

# cd /usr/src

(unless, of course, your source code is elsewhere, in which case change to that directory instead).

To rebuild the world you use the make(1) command. This command reads instructions from the Makefile, which describes how the programs that comprise FreeBSD should be rebuilt, the order in which they should be built, and so on.

The general format of the command line you will type is as follows:

# make -x -DVARIABLE target

In this example, -x is an option that you would pass to make(1). See the make(1) manual page for an example of the options you can pass.

-DVARIABLE passes a variable to the Makefile. The behavior of the Makefile is controlled by these variables. These are the same variables as are set in /etc/make.conf, and this provides another way of setting them.

# make -DNO_PROFILE target

is another way of specifying that profiled libraries should not be built, and corresponds with the

NO_PROFILE=    true     #    Avoid compiling profiled libraries

line in /etc/make.conf.

target tells make(1) what you want to do. Each Makefile defines a number of different “targets”, and your choice of target determines what happens.

Some targets are listed in the Makefile, but are not meant for you to run. Instead, they are used by the build process to break out the steps necessary to rebuild the system into a number of sub-steps.

Most of the time you will not need to pass any parameters to make(1), and so your command like will look like this:

# make target

Where target will be one of many build options. The first target should always be buildworld.

As the names imply, buildworld builds a complete new tree under /usr/obj, and installworld, another target, installs this tree on the current machine.

Having separate options is very useful for two reasons. First, it allows you to do the build safe in the knowledge that no components of your running system will be affected. The build is “self hosted”. Because of this, you can safely run buildworld on a machine running in multi-user mode with no fear of ill-effects. It is still recommended that you run the installworld part in single user mode, though.

Secondly, it allows you to use NFS mounts to upgrade multiple machines on your network. If you have three machines, A, B and C that you want to upgrade, run make buildworld and make installworld on A. B and C should then NFS mount /usr/src and /usr/obj from A, and you can then run make installworld to install the results of the build on B and C.

Although the world target still exists, you are strongly encouraged not to use it.


# make buildworld

It is possible to specify a -j option to make which will cause it to spawn several simultaneous processes. This is most useful on multi-CPU machines. However, since much of the compiling process is IO bound rather than CPU bound it is also useful on single CPU machines.

On a typical single-CPU machine you would run:

# make -j4 buildworld

make(1) will then have up to 4 processes running at any one time. Empirical evidence posted to the mailing lists shows this generally gives the best performance benefit.

If you have a multi-CPU machine and you are using an SMP configured kernel try values between 6 and 10 and see how they speed things up. Timings

Many factors influence the build time, but fairly recent machines may only take a one or two hours to build the FreeBSD-STABLE tree, with no tricks or shortcuts used during the process. A FreeBSD-CURRENT tree will take somewhat longer.

25.7.9 Compile and Install a New Kernel

To take full advantage of your new system you should recompile the kernel. This is practically a necessity, as certain memory structures may have changed, and programs like ps(1) and top(1) will fail to work until the kernel and source code versions are the same.

The simplest, safest way to do this is to build and install a kernel based on GENERIC. While GENERIC may not have all the necessary devices for your system, it should contain everything necessary to boot your system back to single user mode. This is a good test that the new system works properly. After booting from GENERIC and verifying that your system works you can then build a new kernel based on your normal kernel configuration file.

On FreeBSD it is important to build world before building a new kernel.

Note: If you want to build a custom kernel, and already have a configuration file, just use KERNCONF=MYKERNEL like this:

# cd /usr/src
# make buildkernel KERNCONF=MYKERNEL
# make installkernel KERNCONF=MYKERNEL

Note that if you have raised kern.securelevel above 1 and you have set either the noschg or similar flags to your kernel binary, you might find it necessary to drop into single user mode to use installkernel. Otherwise you should be able to run both these commands from multi user mode without problems. See init(8) for details about kern.securelevel and chflags(1) for details about the various file flags.

25.7.10 Reboot into Single User Mode

You should reboot into single user mode to test the new kernel works. Do this by following the instructions in Section 25.7.6.

25.7.11 Install the New System Binaries

You should now use installworld to install the new system binaries.


# cd /usr/src
# make installworld

Note: If you specified variables on the make buildworld command line, you must specify the same variables in the make installworld command line. This does not necessarily hold true for other options; for example, -j must never be used with installworld.

For example, if you ran:

# make -DNO_PROFILE buildworld

you must install the results with:

# make -DNO_PROFILE installworld

otherwise it would try to install profiled libraries that had not been built during the make buildworld phase.

25.7.12 Update Files Not Updated by make installworld

Remaking the world will not update certain directories (in particular, /etc, /var and /usr) with new or changed configuration files.

The simplest way to update these files is to use mergemaster(8), though it is possible to do it manually if you would prefer to do that. Regardless of which way you choose, be sure to make a backup of /etc in case anything goes wrong. mergemaster

Contributed by Tom Rhodes.

The mergemaster(8) utility is a Bourne script that will aid you in determining the differences between your configuration files in /etc, and the configuration files in the source tree /usr/src/etc. This is the recommended solution for keeping the system configuration files up to date with those located in the source tree.

To begin simply type mergemaster at your prompt, and watch it start going. mergemaster will then build a temporary root environment, from / down, and populate it with various system configuration files. Those files are then compared to the ones currently installed in your system. At this point, files that differ will be shown in diff(1) format, with the + sign representing added or modified lines, and - representing lines that will be either removed completely, or replaced with a new line. See the diff(1) manual page for more information about the diff(1) syntax and how file differences are shown.

mergemaster(8) will then show you each file that displays variances, and at this point you will have the option of either deleting the new file (referred to as the temporary file), installing the temporary file in its unmodified state, merging the temporary file with the currently installed file, or viewing the diff(1) results again.

Choosing to delete the temporary file will tell mergemaster(8) that we wish to keep our current file unchanged, and to delete the new version. This option is not recommended, unless you see no reason to change the current file. You can get help at any time by typing ? at the mergemaster(8) prompt. If the user chooses to skip a file, it will be presented again after all other files have been dealt with.

Choosing to install the unmodified temporary file will replace the current file with the new one. For most unmodified files, this is the best option.

Choosing to merge the file will present you with a text editor, and the contents of both files. You can now merge them by reviewing both files side by side on the screen, and choosing parts from both to create a finished product. When the files are compared side by side, the l key will select the left contents and the r key will select contents from your right. The final output will be a file consisting of both parts, which can then be installed. This option is customarily used for files where settings have been modified by the user.

Choosing to view the diff(1) results again will show you the file differences just like mergemaster(8) did before prompting you for an option.

After mergemaster(8) is done with the system files you will be prompted for other options. mergemaster(8) may ask if you want to rebuild the password file and will finish up with an option to remove left-over temporary files. Manual Update

If you wish to do the update manually, however, you cannot just copy over the files from /usr/src/etc to /etc and have it work. Some of these files must be “installed” first. This is because the /usr/src/etc directory is not a copy of what your /etc directory should look like. In addition, there are files that should be in /etc that are not in /usr/src/etc.

If you are using mergemaster(8) (as recommended), you can skip forward to the next section.

The simplest way to do this by hand is to install the files into a new directory, and then work through them looking for differences.

Backup Your Existing /etc: Although, in theory, nothing is going to touch this directory automatically, it is always better to be sure. So copy your existing /etc directory somewhere safe. Something like:

# cp -Rp /etc /etc.old

-R does a recursive copy, -p preserves times, ownerships on files and suchlike.

You need to build a dummy set of directories to install the new /etc and other files into. /var/tmp/root is a reasonable choice, and there are a number of subdirectories required under this as well.

# mkdir /var/tmp/root
# cd /usr/src/etc
# make DESTDIR=/var/tmp/root distrib-dirs distribution

This will build the necessary directory structure and install the files. A lot of the subdirectories that have been created under /var/tmp/root are empty and should be deleted. The simplest way to do this is to:

# cd /var/tmp/root
# find -d . -type d | xargs rmdir 2>/dev/null

This will remove all empty directories. (Standard error is redirected to /dev/null to prevent the warnings about the directories that are not empty.)

/var/tmp/root now contains all the files that should be placed in appropriate locations below /. You now have to go through each of these files, determining how they differ with your existing files.

Note that some of the files that will have been installed in /var/tmp/root have a leading “.”. At the time of writing the only files like this are shell startup files in /var/tmp/root/ and /var/tmp/root/root/, although there may be others (depending on when you are reading this). Make sure you use ls -a to catch them.

The simplest way to do this is to use diff(1) to compare the two files:

# diff /etc/shells /var/tmp/root/etc/shells

This will show you the differences between your /etc/shells file and the new /var/tmp/root/etc/shells file. Use these to decide whether to merge in changes that you have made or whether to copy over your old file.

Name the New Root Directory (/var/tmp/root) with a Time Stamp, so You Can Easily Compare Differences Between Versions: Frequently rebuilding the world means that you have to update /etc frequently as well, which can be a bit of a chore.

You can speed this process up by keeping a copy of the last set of changed files that you merged into /etc. The following procedure gives one idea of how to do this.

  1. Make the world as normal. When you want to update /etc and the other directories, give the target directory a name based on the current date. If you were doing this on the 14th of February 1998 you could do the following:

    # mkdir /var/tmp/root-19980214
    # cd /usr/src/etc
    # make DESTDIR=/var/tmp/root-19980214 \
        distrib-dirs distribution
  2. Merge in the changes from this directory as outlined above.

    Do not remove the /var/tmp/root-19980214 directory when you have finished.

  3. When you have downloaded the latest version of the source and remade it, follow step 1. This will give you a new directory, which might be called /var/tmp/root-19980221 (if you wait a week between doing updates).

  4. You can now see the differences that have been made in the intervening week using diff(1) to create a recursive diff between the two directories:

    # cd /var/tmp
    # diff -r root-19980214 root-19980221

    Typically, this will be a much smaller set of differences than those between /var/tmp/root-19980221/etc and /etc. Because the set of differences is smaller, it is easier to migrate those changes across into your /etc directory.

  5. You can now remove the older of the two /var/tmp/root-* directories:

    # rm -rf /var/tmp/root-19980214
  6. Repeat this process every time you need to merge in changes to /etc.

You can use date(1) to automate the generation of the directory names:

# mkdir /var/tmp/root-`date "+%Y%m%d"`

25.7.13 Rebooting

You are now done. After you have verified that everything appears to be in the right place you can reboot the system. A simple shutdown(8) should do it:

# shutdown -r now

25.7.14 Finished

You should now have successfully upgraded your FreeBSD system. Congratulations.

If things went slightly wrong, it is easy to rebuild a particular piece of the system. For example, if you accidentally deleted /etc/magic as part of the upgrade or merge of /etc, the file(1) command will stop working. In this case, the fix would be to run:

# cd /usr/src/usr.bin/file
# make all install

25.7.15 Questions Do I need to re-make the world for every change? My compile failed with lots of signal 11 (or other signal number) errors. What has happened? Can I remove /usr/obj when I have finished? Can interrupted builds be resumed? How can I speed up making the world? What do I do if something goes wrong? Do I need to re-make the world for every change?

There is no easy answer to this one, as it depends on the nature of the change. For example, if you just ran Subversion, and it has shown the following files as being updated:


it probably is not worth rebuilding the entire world. You could just go to the appropriate sub-directories and make all install, and that is about it. But if something major changed, for example src/lib/libc/stdlib then you should either re-make the world, or at least those parts of it that are statically linked (as well as anything else you might have added that is statically linked).

At the end of the day, it is your call. You might be happy re-making the world every fortnight say, and let changes accumulate over that fortnight. Or you might want to re-make just those things that have changed, and be confident you can spot all the dependencies.

And, of course, this all depends on how often you want to upgrade, and whether you are tracking FreeBSD-STABLE or FreeBSD-CURRENT. My compile failed with lots of signal 11 (or other signal number) errors. What has happened?

This is normally indicative of hardware problems. (Re)making the world is an effective way to stress test your hardware, and will frequently throw up memory problems. These normally manifest themselves as the compiler mysteriously dying on receipt of strange signals.

A sure indicator of this is if you can restart the make and it dies at a different point in the process.

In this instance there is little you can do except start swapping around the components in your machine to determine which one is failing. Can I remove /usr/obj when I have finished?

The short answer is yes.

/usr/obj contains all the object files that were produced during the compilation phase. Normally, one of the first steps in the make buildworld process is to remove this directory and start afresh. In this case, keeping /usr/obj around after you have finished makes little sense, and will free up a large chunk of disk space (currently about 2 GB).

However, if you know what you are doing you can have make buildworld skip this step. This will make subsequent builds run much faster, since most of the sources will not need to be recompiled. The flip side of this is that subtle dependency problems can creep in, causing your build to fail in odd ways. This frequently generates noise on the FreeBSD mailing lists, when one person complains that their build has failed, not realizing that it is because they have tried to cut corners. Can interrupted builds be resumed?

This depends on how far through the process you got before you found a problem.

In general (and this is not a hard and fast rule) the make buildworld process builds new copies of essential tools (such as gcc(1), and make(1)) and the system libraries. These tools and libraries are then installed. The new tools and libraries are then used to rebuild themselves, and are installed again. The entire system (now including regular user programs, such as ls(1) or grep(1)) is then rebuilt with the new system files.

If you are at the last stage, and you know it (because you have looked through the output that you were storing) then you can (fairly safely) do:

… fix the problem …
# cd /usr/src
# make -DNO_CLEAN all

This will not undo the work of the previous make buildworld.

If you see the message:

Building everything..

in the make buildworld output then it is probably fairly safe to do so.

If you do not see that message, or you are not sure, then it is always better to be safe than sorry, and restart the build from scratch. How can I speed up making the world?

  • Run in single user mode.

  • Put the /usr/src and /usr/obj directories on separate file systems held on separate disks. If possible, put these disks on separate disk controllers.

  • Better still, put these file systems across multiple disks using the ccd(4) (concatenated disk driver) device.

  • Turn off profiling (set “NO_PROFILE=true” in /etc/make.conf). You almost certainly do not need it.

  • Pass the -jn option to make(1) to run multiple processes in parallel. This usually helps regardless of whether you have a single or a multi processor machine.

  • The file system holding /usr/src can be mounted (or remounted) with the noatime option. This prevents the file system from recording the file access time. You probably do not need this information anyway.

    # mount -u -o noatime /usr/src

    Warning: The example assumes /usr/src is on its own file system. If it is not (if it is a part of /usr for example) then you will need to use that file system mount point, and not /usr/src.

  • The file system holding /usr/obj can be mounted (or remounted) with the async option. This causes disk writes to happen asynchronously. In other words, the write completes immediately, and the data is written to the disk a few seconds later. This allows writes to be clustered together, and can be a dramatic performance boost.

    Warning: Keep in mind that this option makes your file system more fragile. With this option there is an increased chance that, should power fail, the file system will be in an unrecoverable state when the machine restarts.

    If /usr/obj is the only thing on this file system then it is not a problem. If you have other, valuable data on the same file system then ensure your backups are fresh before you enable this option.

    # mount -u -o async /usr/obj

    Warning: As above, if /usr/obj is not on its own file system, replace it in the example with the name of the appropriate mount point. What do I do if something goes wrong?

Make absolutely sure your environment has no extraneous cruft from earlier builds. This is simple enough.

# chflags -R noschg /usr/obj/usr
# rm -rf /usr/obj/usr
# cd /usr/src
# make cleandir
# make cleandir

Yes, make cleandir really should be run twice.

Then restart the whole process, starting with make buildworld.

If you still have problems, send the error and the output of uname -a to FreeBSD general questions mailing list. Be prepared to answer other questions about your setup!