Tag: linux

Duplicate IP issues with Linux and virtual machine cloning.

TL;DR – Some modern Linux distributions use a newer method of identification which, when combined with DHCP can result in duplicate IP addresses when cloning VMs, even when the VMs have unique MAC addresses.

To resolve, do the following ( remove file, run the systemd-machine-id-setup command, reboot):

When hypervisor management tools make clones of virtual machines, the tools usually make sure to create a unique MAC address for every clone. Combined with DHCP, this is normally enough to boot the clones and have them receive a unique IP. Recently, when I cloned several Bitnami guest VMs which are based on Debian, I started to get duplicate IP addresses on the clones. The issue can be resolved manually by following the above procedure.

To create a VM template to clone from which will generate a new machine-id for very clone, simply create an empty /etc/machine-id file (do not rm the file, otherwise the machine-id will not be generated)

The machine-id man page is a well written explanation of the implementation and motivation.

Performance gains for postgres on Linux with hugepages

For this experiment I am using Postgres v11 on Linux 3.10 kernel. The goal was to see what gains can be made from using hugepages. I use the “built in” benchmark pgbench to run a simple set of queries.

Since I am interested in only the gains from hugepages I chose to use the “-S” parameter to pgbench which means perform only the “select” statements. Obviously this masks any costs that might be seen when dirtying hugepages – but it kept the experiment from having to be concerned with writing to the filesystem.

Experiment

The workstation has 32GB of memory
Postgres is given 16GB of memory using the parameter


pgbench creates a ~7.4gb database using a scale-factor of 500

Run the experiment like this

Result

Default : No hugepages :
tps = 62190.452850 (excluding connections establishing)

2MB Hugepages
tps = 66864.410968 (excluding connections establishing)
+7.5% over default

1GB Hugepages
tps = 69702.358303 (excluding connections establishing)
+12% over default

Enabling hugepages

Getting the default hugepages is as easy as entering a value into /etc/sysctl.conf. To allow for 16GB of hugepages I used the value of 8400, followed by “sysctl -p”

To get 1GB hugepages, the kernel has to have it configured during boot e.g.

Then reboot the kernel

I used these excellent resources
How to modify the kernel command line
How to enable hugepages
and this great video on Linux virtual memory



The return of misaligned IO

We have started seeing misaligned partitions on Linux guests runnning certain HDFS distributions.  How these partitions became mis-aligned is a bit of a mystery, because the only way I know how to do this on Linux is to create a partition using old DOS format like this (using -c=dos  and -u=cylinders)  Continue reading “The return of misaligned IO”