Add more swap space to a running Linux system

Today I was trying to install Oracle XE to a Centos 5 installation using this guide. While installing the packages i noticed that the Oracle XE Database was unable to install. The error message was quite clear:

This system does not meet the minimum requirements for swap space.  Based on
the amount of physical memory available on the system, Oracle Database 10g
Express Edition requires 1024 MB of swap space. This system has 509 MB
of swap space.  Configure more swap space on the system and retry the installation.
error: %pre(oracle-xe-univ-10.2.0.1-1.0.i386) scriptlet failed, exit status 1
error:   install: %pre scriptlet failed (2), skipping oracle-xe-univ-10.2.0.1-1.0

My solution was to adding some temporary swap space to this system, which is really easy to do:

Step 1. Create an empty file called /swapfile from about 600MB

root@bootux:~# dd if=/dev/zero of=/swapfile bs=1024000 count=600
600+0 records in
600+0 records out
614400000 bytes (614 MB) copied, 5.85153 seconds, 105 MB/

Step 2. Format the new file to make it a swap file

root@bootux:~# mkswap /swapfile
Setting up swapspace version 1, size = 614395 kB

Step 3. Enable the new swapfile. Only the swapon command is needed, but with the free command you can clearly see the swap space is made available to the system.

root@bootux:~# free -m | grep Swap
Swap:          509          0        509
root@bootux:~# swapon /swapfile
root@bootux:~# free -m | grep Swap
Swap:         1095          0       1095

Installation can now continue :)

Easily create a RAM-disk in Linux

This command creates a 2GB RAM-disk and mounts it to /tmp/space.

$ mkdir /tmp/space
$ sudo mount -t tmpfs tmpfs /tmp/space -o size=2G,nr_inodes=200k,mode=01777

You should now have an extra 2GB storage device.

 $ df -h | grep space
$ tmpfs                 2.0G     0  2.0G   0% /tmp/space

You access the temporarily device by it's mountpoint /tmp/space. Needless to say, once you unmount this volume (for instance, with a reboot), the data on it will be gone. Use with care! :) Tested on Ubuntu, should work with any 2.4 or 2.6 kernel...

Clearing Linux' Memory Cache

Note to self: Here are some commands you can use to clear the memory cache on a Linux system. The following command sequence will do the following: - display the memory stats - flush filesystem buffers - drop cached memory - display the memory stats again

$ free $ sudo sync $ sudo echo 3 | sudo tee /proc/sys/vm/drop_caches $ free

Or, if you like one-liners:

free; sudo sync; sudo echo 3 | sudo tee /proc/sys/vm/drop_caches; free

This is the ouput that i got. The values in bold are my commands, if you look at the underlined values, you will see the memory drop.

linux01:~ $ free
             total       used       free     shared    buffers     cached
Mem:       2061568    2029968      31600          0     429284    1247252
-/+ buffers/cache:     353432    1708136
Swap:      1036152        104    1036048
linux01:~ $ sudo sync
linux01:~ $ sudo echo 3 | sudo tee /proc/sys/vm/drop_caches
3
linux01:~ $ free
             total       used       free     shared    buffers     cached
Mem:       2061568     724828    1336740          0        340     392092
-/+ buffers/cache:     332396    1729172
Swap:      1036152        104    1036048

I tested this on SLES10 and Centos5 and Ubuntu8.04. My Ubuntu 6.06 and 7.10 boxes do not seem to have the file /proc/sys/vm/drop_caches . Sources: 1 and 2.

Enable Time Sync from inside a Linux VM

Note to self: Command to enable VMware Time Synchronisation on a Linux VM without a GUI (with VMware Tools installed):

vmware-guestd --cmd "vmx.set_option synctime 1 0"

To disable it you have to turn around the last two parameters, like this:

vmware-guestd --cmd "vmx.set_option synctime 0 1"

If you want to apply a setting that is already current you will get the message:

"Invalid old value"

This is tested on RedHat Enterprise Linux 5 and SUSE Linux Enterprise Server 10. Source: Timekeeping best practices for Linux on kb.vmware.com

xampp-dsl: A Damn Small LAMP Appliance

Today I ran across a nice Virtual Appliance that consists of the well-known xampp stack (version 1.6.1) installed on top of the tiny Linux distro Damn Small Linux (DSL 3.3). You can get the appliance here . The download weighs in at 154MB and the unpacked size is 379MB.

KQemu on Ubuntu Gutsy Gibbon

QEMU is an open source processor emulator in which you can run Virtual Machines. It comes with a kernel module called KQemu which accelerates the VM's to 'near native' speeds. Due to some licensing issues KQemu is not distributed and installed with QEMU, though it is easily available in Ubuntu and a lot of other operating systems. This message appears when starting QEMU while KQemu is not loaded:

Could not open '/dev/kqemu' - QEMU acceleration layer not activated

This howto shows how to get KQemu running on Ubuntu 7.10 (Gutsy Gibbon) First install the following packages:

sudo apt-get install kqemu-source kqemu-common build-essential

Then run the following commands:

sudo module-assistant prepare kqemu sudo module-assistant auto-install kqemu

If everything went right (so no errors occurred) you can load the module with this command:

sudo modprobe kqemu

Now check if the module is loaded correctly. If you get a line of output starting with the word kqemu it means the module is loaded properly:

lsmod | grep qemu

To use this module as a user the right permission need to be set:

sudo chmod a+rw /dev/kqemu

You QEMU VM's should now run without the warning above and run a whole lot faster!