VM Best Practices

This page is to provide some guidance based on user experiences with Compute Canada's cloud instances. Your mileage may vary.

Volumes

Data volumes

It is very difficult to expand a root volume, so for any VM which does not have bounded storage requirements, consider creating a second volume for data. If more space is needed, and your allocation has enough remaining volume storage, it is fairly straightforward to expand the data volume using OpenStack and then expanding the logical volume, if any, and filesystem within your VM.

Maximum volumes per VM

Avoid attaching more than three volumes to a VM. This has been observed to lead to kernel timeouts which can affect any disk operations on those volumes and may have a cascading effect, and can render the VM effectively inoperative. While using a data volume is advisable in some cases (see above), use only one and you can carve it up into multiple filesystems inside your VM, and expand it as necessary.

This has been observed on Arbutus (west.cloud.computecanada.ca) by multiple users, although it may be somewhat dependent on volume size. In one case, 3 x 100GB volumes attached to a p4-6gb VM caused kernel timeouts whenever disk operations of any magnitude were attempted. It was very difficult to copy more than 500MB of data between two filesystems, for example.

Test of a similar situation was performed on East Cloud and with 4 x 100GB data volumes (along with the root volume), the same OS and a similar VM flavour, the issue did not occur. This VM had more memory (15 GB instead of 6 GB) but high memory usage was not observed in the Arbutus case and this isn't believed to be a factor.