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Hacking BSD, Part 1
Pages: 1, 2

Hack #21. Manage Temporary Files and Swap Space

Add more temporary or swap space without repartitioning.

When you install any operating system, it's important to allocate sufficient disk space to hold temporary and swap files. Ideally, you already know the optimum sizes for your system so you can partition your disk accordingly during the install. However, if your needs change or you wish to optimize your initial choices, your solution doesn't have to be as drastic as a repartition - and reinstall - of the system.

TIP: man tuning has some practical advice for guesstimating the appropriate size of swap and your other partitions.

Clearing /tmp

Unless you specifically chose otherwise when you partitioned your disk, the installer created a /tmp filesystem for you:



% grep tmp /etc/fstab
/dev/ad0s1e    /tmp    ufs    rw    2    2

% df -h /tmp

Filesystem    Size   Used  Avail Capacity  Mounted on
/dev/ad0s1e   252M   614K   231M     0%    /tmp

Here I searched /etc/fstab for the /tmp filesystem. This particular filesystem is 256 MB in size. Only a small portion contains temporary files.

TIP: The df (disk free) command will always show you a number lower than the actual partition size. This is because eight percent of the filesystem is reserved to prevent users from inadvertently overflowing a filesystem. See man tunefs for details.

It's always a good idea to clean out /tmp periodically so it doesn't overflow with temporary files. Consider taking advantage of the built-in periodic script /etc/periodic/daily/110.clean-tmps [Hack #20] .

You can also clean out /tmp when the system reboots by adding this line to /etc/rc.conf:

clear_tmp_enable="YES"

Moving /tmp to RAM

Another option is to move /tmp off of your hard disk and into RAM. This has the built-in advantage of automatically clearing the filesystem when you reboot, since the contents of RAM are volatile. It also offers a performance boost, since RAM access time is much faster than disk access time.

Before moving /tmp, ensure you have enough RAM to support your desired /tmp size. This command will show the amount of installed RAM:

% dmesg | grep memory
real memory  = 335462400 (319 MB)
avail memory = 320864256 (306 MB)

Also check that your kernel configuration file contains device md (or memory disk). The GENERIC kernel does; if you've customized your kernel, double-check that you still have md support:

% grep -w md /usr/src/sys/i386/conf/CUSTOM

device        md    # Memory "disks"

Changing the /tmp line in /etc/fstab as follows will mount a 64 MB /tmp in RAM:

md /tmp mfs rw,-s64m 0 0

Next, unmount /tmp (which is currently mounted on your hard drive) and remount it using the new entry in /etc/fstab:

# umount /tmp
# mount /tmp

# df -h /tmp
Filesystem    Size   Used  Avail Capacity  Mounted on
/dev/md0       63M   8.0K    58M     0%    /tmp

Notice that the filesystem is now md0, the first memory disk, instead of ad0s1e, a partition on the first IDE hard drive.

Creating a Swap File on Disk

Swap is different than /tmp. It's not a storage area for temporary files; instead, it is an area where the filesystem swaps data between RAM and disk. A sufficient swap size can greatly increase the performance of your filesystem. Also, if your system contains multiple drives, this swapping process will be much more efficient if each drive has its own swap partition.

The initial install created a swap filesystem for you:

% grep swap /etc/fstab
/dev/ad0s1b    none     swap    sw    0    0

% swapinfo

Device          1K-blocks     Used    Avail Capacity  Type
/dev/ad0s1b        639688       68   639620     0%    Interleaved

Note that the swapinfo command displays the size of your swap files. If you prefer to see that output in MB, try the swapctl command with the -lh flags (which make the listing more human):

% swapctl -lh
Device:       1048576-blocks      Used:
/dev/ad0s1b          624          0

To add a swap area, first determine which area of disk space to use. For example, you may want to place a 128 MB swapfile on /usr. You'll first need to use dd to create this as a file full of null (or zero) bytes. Here I'll create a 128 MB swapfile as /usr/swap0:

# dd if=/dev/zero of=/usr/swap0 bs=1024k count=128

128+0 records in
128+0 records out
134217728 bytes transferred in 4.405036 secs (30469156 bytes/sec)

Next, change the permissions on this file. Remember, you don't want users storing data here; this file is for the filesystem:

# chmod 600 /usr/swap0

Since this is really a file on an existing filesystem, you can't mount your swapfile in /etc/fstab. However, you can tell the system to find it at boot time by adding this line to /etc/rc.conf:

swapfile="/usr/swap0"

To start using the swapfile now without having to reboot the system, use mdconfig:

# mdconfig -a -t vnode -f /usr/swap0 -u 1 && swapon /dev/md1

The -a flag attaches the memory disk. -t vnode marks that the type of swap is a file, not a filesystem. The -f flag sets the name of that file: /usr/swap0.

The unit number -u 1 must match the name of the memory disk /dev/md1. Since this system already has /tmp mounted on /dev/md0, I chose to mount swap on /dev/md1. && swapon tells the system to enable that swap device, but only if the mdconfig command succeeded.

swapctl should now show the new swap partition:

% swapctl -lh
Device:       1048576-blocks      Used:
/dev/ad0s1b          624          0
/dev/md1             128          0

Monitoring Swap Changes

Whenever you make changes to swap or are considering increasing swap, use systat to monitor how your swapfiles are being used in real time:

% systat -swap

The output will show the names of your swap areas and how much of each is currently in use. It will also include a visual indicating what percentage of swap contains data.

OpenBSD Differences

You can make this hack work on OpenBSD, as long as you remember that the RAM disk device is rd and its configuration tool is rdconfig. Read the relevant manpages, and you'll be hacking away.

See Also

Dru Lavigne is a network and systems administrator, IT instructor, author and international speaker. She has over a decade of experience administering and teaching Netware, Microsoft, Cisco, Checkpoint, SCO, Solaris, Linux, and BSD systems. A prolific author, she pens the popular FreeBSD Basics column for O'Reilly and is author of BSD Hacks and The Best of FreeBSD Basics.


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