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.
man tuninghas some practical advice for guesstimating the appropriate size of swap and your other partitions.
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.
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 tunefsfor 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:
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
(or memory disk). The
GENERIC kernel does; if you've customized
your kernel, double-check that you still have
% 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,
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
command with the
-lh flags (which make the listing more
% 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
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
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:
To start using the swapfile now without having to reboot the system, use
# mdconfig -a -t vnode -f /usr/swap0 -u 1 && swapon /dev/md1
-a flag attaches the memory disk.
-t vnode marks
that the type of swap is a file, not a filesystem. The
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
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
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.
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
Read the relevant manpages, and you'll be hacking away.
man tuning (practical advice on /tmp and swap)
- The BSD Handbook entry on adding swap
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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