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):
# rm /etc/machine-id
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)
# echo "" | tee /etc/machine-id
The machine-id man page is a well written explanation of the implementation and motivation.
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.
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
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”