Ave Maria
Posted: 2012-12-06 19:01:31 by Alasdair Keyes
What does your database choice say about you?
Well, I don't know if they actually do say anything, however I've been using MySQL for a long time. If I need a database for a project, either for work and personal I fall back to MySQL, it's like the comfy old pair of shoes that you always wear because you know where you are with them and it always feels comfortable.
I dabbled a bit with Oracle at uni and dipped my toe in the Postgres waters a couple of years ago to see what the fuss was about. Both seemed very functional, however my only reason not to switch was that I was comfortable with MySQL. As a developer I know the SQL syntax to do pretty much most things I'd want to do and as a Sys Admin, I know how to set it up in multiple configurations, upgrade it, manage it and debug issues. Beyond that I'd got a lot of projects currently running on it and no one needs the headache of changing a core part of a system such as the database, as the saying goes... "If it ain't broke..."
Those who follow such things will be aware that MySQL has had a somewhat rocky history over the past few years with, being bought in 2008 by Sun and then acquired by Oracle a couple of years later. This has worried a lot of people, after all what would Oracle want with a free Database solution that might take business away from their high-end, uber-expensive prized product?
I have to say, I'm also a little concerned so I've been following MariaDB with some interest recently. MariaDB is a drop-in replacement for MySQL forked from the MySQL community version. It also has a few extensions and little extras of it's own should you want to use them but still maintains the backwards compatibility with MySQL. They very nicely also provide repositories for some of the main Linux distributions CentOS, Debian, Fedora, Redhat and Ubuntu, so you can get the latest and greatest versions and bug fixes all at the cost of adding a repository into your package management system.
I'm not sure I want to migrate my work systems to it yet, however I think I'll be giving it a go for some of my new projects.
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Coding by famous Authors
Posted: 2012-08-24 15:24:49 by Alasdair Keyes
Here's a bit of light-hearted techy humour for those who like to code..
http://byfat.xxx/if-hemingway-wrote-javascript
I'm quite taken with the Shakespeare version.
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Sendmail takes a long time to start
Posted: 2012-07-26 13:39:39 by Alasdair Keyes
I had someone come to me today with a CentOS 5.x machine that was taking upwards of 10 minutes to get to the login prompt.
When I rebooted the box, It hung for a long time on Sendmail and the Sendmail sm-client services
Starting Sendmail:
Between the two of them they took over 8 minutes to start. Helpfully /var/log/messages and /var/log/maillog didn't have any entries about why it took so long. After a bit of poking it turns out that he had changed the hostname from kvmhost01.local to kvmhost01.testlocal but hadn't updated /etc/hosts so sendmail couldn't find the IP associated with the hostname.
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 kvmhost01.testlocal localhost.localdomain localhost
::1 localhost6.localdomain6 localhost6
It's only a small thing but obviously makes a big difference to Sendmail.
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Network Ranges By Country/Continent
Posted: 2012-04-05 12:46:45 by Alasdair Keyes
Whilst investigating the source of some network attacks this morning, I came across the following website
http://www.countryipblocks.net
It provides lists of Network blocks by Country/Continent, if you notice a large number of attacks from a specific geographical area, you can find other IP blocks from the same area to add firewall rules.
According to the website, the data may become a paid-for service soon, so you may want to get the information while it's free!
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KVM Console Not working
Posted: 2011-11-25 13:09:11 by Alasdair Keyes
I've noticed that with KVM virtual machines, the kvm console <domainid> doesn't appear to work, when running you get
# virsh console MyVM
Connected to domain MyVM
Escape character is ^]
Hitting enter gives you nothing It appears that this isn't bug with KVM, rather the Guest OS isn't aware that it should start up a ttyS for KVM to connect to. When starting the machine, enter into grub and add the following to the kernel options...
console=ttyS0,115200
Then when the virtual machine starts you can connect again using virsh and hit enter a couple of times
[root@inth1-vdc-lvh01 ~]# virsh console MyVM
Connected to domain MyVM
Escape character is ^]
CentOS Linux release 6.0 (Final)
Kernel 2.6.32-71.el6.x86_64 on an x86_64
centos-6-0 login:
Voila... Obviously this isn't going to work for Windows :)
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BASH v4 Regular Expressions
Posted: 2011-03-05 14:09:48 by Alasdair Keyes
I've recently built a new backup server using NILFS for it's handy snapshotting capability. I'll get onto that in another post, but because NILFS support was only added into the Linux 2.6.30 kernel, our current CentOS 5 build's don't have support for it as they only run 2.6.18.
To overcome this, I built the new box up with Ubuntu 10.10 Server. However, it appears that I'd never written any BASH scripts using regular expressions in Ubuntu. I copied my custom backup scripts from our old box to the new backup server and ran them and it errored on some path name checks I performed. Upon investigation it appears that BASH v4's regular expression handling isn't backwards compatible with version 3. Specifically the use of the $ character as the End of Line character
As a test...
#!/bin/bash
TESTDIR="/tmp/a/";
if [[ $TESTDIR =~ '/$' ]]; then
echo "$TESTDIR has a trailing slash";
else
echo "$TESTDIR has no trailing slash";
fi
The output is
/tmp/a/ has no trailing slash
Which doesn't seem too clever to me. After some looking through the bash documentation I found out about the 'shopt' builtin which allows you to change additional shell behavior.
Long story short, so I don't have to go round changing all the regex in my code, if I add something like this to the top of all my scripts, they can port between Bash v3 and v4 seamlessly...
#!/bin/bash
BASH_VER=`/bin/bash --version | grep 'version 4'`;
if [ -n "$BASH_VER" ]; then
echo "Detected BASH v4 - Switching on Version 3 compatibility"
shopt -s compat31
else
echo "Not Detected BASH v4"
fi
TESTDIR="/tmp/a/";
if [[ $TESTDIR =~ '/$' ]]; then
echo "$TESTDIR has a trailing slash";
else
echo "$TESTDIR has no trailing slash";
fi
Produces more expected output
Detected BASH v4 - Switching on Version 3 compatibility
/tmp/a/ has a trailing slash
Or if you're not bothered about nice output, shorten the check to..
if [ -n "`/bin/bash --version | grep 'version 4'`" ]; then
shopt -s compat31
fi
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FLOSS and CentOS
Posted: 2010-11-18 16:44:24 by Alasdair Keyes
Earlier today I watched this podcast from FLOSS (Free Libre Open Source Software) on the TWiT Network whose special guest was Karanbir Singh from the CentOS project. Karanbir talked about CentOS, it's start in life, it's relationship with the Redhat Enterprise Linux system and his experiences with the project which were very interesting to listen to.
Being a lead developer on CentOS, Karanbir commits most of the package updates for the CentOS 5 systems I use at both home and work and I get many emails from him via the CentOS Announce list advising of new package commits. In a strange way it was interesting to see the person who I've never met and probably never will but has huge influence over my work life.
If you're interested in hearing about Open Source projects or use CentOS on any machines, I'd highly recommend spending an hour and watching (or listening) to this.
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Centos/Redhat 6
Posted: 2010-11-11 13:00:01 by Alasdair Keyes
Redhat have just released their latest Enterprise Linux offering Redhat EL 6. According to the CentOS twitter feed http://twitter.com/centos it will be 4-6 weeks before CentOS 6 is built and released publicly.
The two biggest changes for me are that exim is now no longer in the Redhat Repository and Xen DomU supported kernels have been dropped in favour of KVM technology.
Exim is the only MTA I really want to use, so I won't be switching to Postfix or sendmail. Thankfully, rebuilding the Latest Exim Source RPM from Redhat 5 on Redhat 6 seems very easy, so it shouldn't cause too much of an issue that I can see. Once CentOS 6 is released, I'll post an article about getting Exim running on 6, so keep posted.
I'm very intrigued to use KVM, I've had very little experience of it, but I've used Xen a lot, building our entire Linux VPS platform around it. The "bonusses" of KVM are looking interesting, especially the memory overcommit facility that's available. If I can get some hardware to test with I'll look at the ease of building up a redundant KVM cluster.
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Jumbo Frames
Posted: 2010-10-01 18:35:44 by Alasdair Keyes
We'd been having some issues with retransmit errors on our iSCSI network, so we enabled jumbo frames to allow frames up to 9000 bytes instead of the standard 1500. The change was very easy to implement...
ifconfig ethx mtu 9000
And then updated the /etc/sysconfig/network-scripts/ifcfg-ethx file to include the line
MTU=9000
(We use CentOS)
Using the Dell SANHQ software showed a marked increase in the speed of large iSCSI transactions, However we discovered that some of our CentOS based Xen hosts were dropping incoming packets on the iSCSI interface
$ ifconfig ethx | grep dropped
RX packets:1883666 errors:0 dropped:12636 overruns:0 frame:0
TX packets:1696267 errors:0 dropped:0 overruns:0 carrier:0
This was a concern as we'd not had any issues with this before, although some dropped packets can be forgiven, in this case we seemed to have problems with our Hosts kernel panicing and rebooting. We couldn't work out what was causing these dropped packets and tried a number of things including increasing the incoming TCP buffer, but to no avail.
After a short time I noticed that the hosts running slightly earlier kernels were getting the packet loss, whereas the later ones were fine, specifically Redhat/CentOS kernels 2.6.18-164.x.x.el5xen were dropping packets, where as the 2.6.18-194.x.x.el5xen kernels were fine.
There's no special fix in this instance, if you're getting s similar issue and running the 2.6.18-164 kernel make sure you do an update. It's also just a reminder to keep your servers and especially kernels, up to date, although software is often only updated for security patches, there are many small fixes that are rolled out that fix little issues like this.
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Parallels Virtualization Engineer
Posted: 2010-08-17 18:53:00 by Alasdair Keyes
At Daily.co.uk we've recently released our Parallels Virtuozzo Based VPS systems. Parallel's container based virtualization is a very low-overhead system which allows a very high ratio of VPS guest OS's per physical hardware node.
As part of this I attended a two-day course plus various online tutorials and exams and I am now a Parallels Certified Virtualization Engineer and Parallels Certified Automation Engineer.
Which is nice.
If you are using Parallels and wish to look at getting certified with their technologies you can check them out at http://www.parallels.com/uk/support/training/
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© Alasdair Keyes
IT Consultancy Services
I'm now available for IT consultancy and software development services - Cloudee LTD.
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