h3x.no » Linux

Yum on Red Hat 5 hangs when using proxy

Tor Henning Ueland — Sun, 05 Feb 2012 20:24:43 +0000

I had a machine that started hanging when running yum update on it. And the only way to actually stop yum then was to kill the process from another shell, pretty strange.

I noticed that it was trying to look up and old proxy server which was not in use anymore, the machine itself got online without issues on other services but then i realised that the file “/etc/sysconfig/rhn/up2date” had a entry for yum proxy settings.

It is also worth checking all files in your “/etc/yum” folder and make sure that none of your repo files have dedicated proxy settings defined since that will override anything from up2date.

When commenting out that proxy setting, yum started behaving again.

Wrong charset for cron

Tor Henning Ueland — Sat, 28 Jan 2012 14:02:41 +0000

I got a issue on a export job i had that was running daily on which the charset of the data exported wer wrong, and this did not happen when i ran the job manually. I noticed that Cron used the charset “POSIX” instead of en_US.UTF-8 which i used. I fixed this by adding “LANG=en_US.UTF-8” to the file /etc/default/locale.

It appears that when the LANG variable is not set in that file, Cron will fall back to using the POSIX charset.

Expanding a kVM disk image

Tor Henning Ueland — Sun, 14 Aug 2011 19:24:31 +0000

Had to expand a KVM virtual machine today. Luckily, that`s pretty straight forward. You simply create a new disk image with the extra size needed, merge it into the original disk and voila. Then you just need to partition in the extra space and you are good to go.

How-to:

1: Halt your virtual machine.

You need to stop your virtual machine before going wild with the drive. Virsh stop , or virsh destroy if it somehow wont stop.

2: Create a disk with the extra space needed:

qemu-img create -f raw 5gig.img 5G

3: Merge it into the disk you are working with

cat 5gig.img >> yourdisk.img

4: Boot up and and partition your drive.

Then start up your virtual machine again with virsh start . If you use Windows server, all you need to do is to visit disk managent, right click your drive with little free space and choose “extend partition”. The job takes seconds and does not require any reboot.

Tuning Ubuntu mdadm RAID5/6

Tor Henning Ueland — Sat, 09 Jul 2011 19:59:58 +0000

If you are using mdadm RAID 5 or 6 with Ubuntu, you might notice that the performance is not all uber all the time. Reason for this is that the default tuning settings for Ubuntu is set to rather motdest values. These can lucikly easily be tuned. I will in this article increase some settings until my read and write performance against my RAID 6 has been improved a lot.

My setup:
CPU: Intel(R) Core(TM)2 Quad CPU Q9300
RAM: 16G
Drives: 11 drives in one RAID6 with drives split over two cheap PCI-E x4 controllers and the motherboard`s internal controller.

I will test my system between each tuning by using dd for read and write testing. Since i have a nice amount of RAM available, i will use a test file of 36G. (bs=16k) Between each test (both read and write), i clear the OS disk cache with the command:

sync;echo 3 > /proc/sys/vm/drop_caches

Tuning stripe_cache_size

stripe_cache_size affects RAM used by mdadm to writing of data. Ubuntu`s default value is 256, you can verify your value by doing:

cat /sys/block/md0/md/stripe_cache_size

And changing it with:

echo *number* > /sys/block/md0/md/stripe_cache_size

Test results with stripe_cache_size=256
- Write performance: 174 MB/s

Not to good, i therefore increased it some levels, each level with result is described below:

Test results with stripe_cache_size=512
- Write performance: 212 MB/s

Test results with stripe_cache_size=1024
- Write performance: 237 MB/s

Test results with stripe_cache_size=2048
- Write performance: 254 MB/s

Test results with stripe_cache_size=4096
- Write performance: 295 MB/s

Test results with stripe_cache_size=8192
- Write performance: 362 MB/s

Test results with stripe_cache_size=16384
- Write performance: 293 MB/s

Test results with stripe_cache_size=32768
- Write performance: 326 MB/s

So, going from 256 to 32K ~doubled my write performance, not bad!

Tuning Read Ahead

Time to change a bit on read ahead, which should impact read performance. Default read ahead value is “1536″, and you can change it with the command:

blockdev --setra *number* /dev/md0

Test results with Read Ahead @ 1536
- Read performance: 717 MB/s

Test results with Read Ahead @ 4096
- Read performance: 746 MB/s

Test results with Read Ahead @ 32768
- Read performance: 731 MB/s

Test results with Read Ahead @ 262144
- Read performance: 697 MB/s

Test results with Read Ahead @ 524288
- Read performance: 630 MB/s

So oposite of the write performance tuning, this actually became worse for most of the settings. So 4096 is the best for my system.

In conclution

This is just an example on how different settings can have rather large impact on a system, both for the better and for the worse. If you are going to tune your system you have to test different setting for yourself and see what works best for your setup. Higher values does not automaticly mean better results. I ended up with “stripe_cache_size=8192″ and “Read Ahead @ 4096″ for my system.

If you want to make sure that your changes is saved when rebooting the system, remember to add these commands (with your values) in /etc/rc.local.

Change hostname on Linode VPS

Tor Henning Ueland — Sat, 14 May 2011 20:35:19 +0000

Linode explains pretty well how to change the hostname of your VPS. But they do not mention that in the latest Ubuntu it is set via Linode`s own DHCP server. So even if you set it via /etc/hostname and in hosts, it will still be overwritten by Linode`s own hostname given to your server.

The solution is to kindly thell DHCPCD to *not* override the hostname you have set, open /etc/default/dhcpcd and alter the following:

1	SET_HOSTNAME='yes'

1	SET_HOSTNAME='no'

Reboot and voila!

Clear disk cache on Linux

Tor Henning Ueland — Fri, 06 May 2011 22:40:11 +0000

I have been doing a bit benchmarking in the previous days, and have then needed to clear the disk cache from RAM without wanting to reboot each time. The command i use for that is:

1	sync;echo 3 > /proc/sys/vm/drop_caches

Which tells the kernel to free pagecache, dentries and inodes.

WordPress: Image could not be processed. Please go back and try again.

Tor Henning Ueland — Sun, 06 Mar 2011 13:30:09 +0000

This is a rather cryptical error message WordPress gives when trying to upload a header image and it fails.

The solution is to make sure your server has php(5)-gd installed.

Ubuntu:

apt-get install php5-gd

Red Hat:

yum install php-gd

jpackages error: Missing Dependency: /usr/bin/rebuild-security-providers

Tor Henning Ueland — Fri, 25 Feb 2011 10:02:27 +0000

Jpackages on Red Hat has a nifty bug that causes dependency errors.

Luckily, somebody has created a fix as a rpm package

wget http://plone.lucidsolutions.co.nz/linux/centos/images/jpackage-utils-compat-el5-0.0.1-1.noarch.rpm
rpm -ivh jpackage-utils-compat-el5-0.0.1-1.noarch.rpm

And then jpackages works.

ImportError: No module named trac

Tor Henning Ueland — Thu, 24 Feb 2011 13:43:55 +0000

When working with a new Trac installation you can bump into the error message “ImportError: No module named trac”. This is usually caused by Trac installation not unzipping all the needed files.

The following one liner should fix the issue:

1	cd /usr/lib/python2.4/site-packages;unzip Trac-0.12.2-py2.4.egg

package subversion-1.6.12-0.1.el5.rf.x86_64 (which is newer than subversion-1.4.2-4.el5_3.1.i386) is already installed

Tor Henning Ueland — Thu, 24 Feb 2011 09:30:23 +0000

Package conflict that can appear some times, a simple fix then is to downgrade first, then upgrade:

yum --downgrade update subversion

yum update subversion