Recently I’ve worked on a project where we deployed a bunch KVM instances. Immediately we noticed horrible IO performance on all the guests instances. In this particular case the hosts and the guests were all Ubuntu 10.04 Lucid and were created with vmbuilder without any special settings using the ubuntu defaults. Here is a sample command similar to what we used to build the kvm images:

vmbuilder kvm ubuntu --suite=lucid --flavour=virtual --arch=amd64 --mirror=http://en.archive.ubuntu.com/ubuntu -o --libvirt=qemu:///system --ip=10.0.0.11 --gw=10.0.0.1 --part=vmbuilder.partition --templates=mytemplates --user=username --pass=password --firstboot=/var/vms/vm1/boot.sh --mem=1024 --hostname=myhost --bridge=br0

Now even if we haven’t tuned anything I would have expected it to perform at least the same level or even better compared with a Xen instance. Still, this was not the case, and the performance was really horrible and any kind of IO bound tasks would effectively lock the instance. Looking into this and trying to understand what was the problem I was able to isolate this issue happening only on instances that had ext4 as the filesystem (the default for lucid), but strangely enough this didn’t happen for an older instance that was build with ext3 (actually a debian lenny instance). All the images build with the above command will use qcow2 sparse format as the default format for the disk.

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I just stumbled across this interesting mysql tuning script called MySQLTuner. Don’t see this (or any other similar script) as the magical tool that will optimize your mysel server… There is no such thing, because there are so many different applications and each will require its specific database tunings.
Still, mysqltuner can be very useful tool in many cases, like for ex:

• it gives a quick overview on your running mysql configuration and some common sense recommendations.
• i really liked how it computes the total amount of memory needed for the current configuration (assuming all global buffers will be used and the maximum number of threads being connected).
• very simple usage: you just have to download and run it.

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