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Storage Classes

OpenStack's storage is backed by Ceph, which is configured to provide multiple storage classes. These storage classes refer to the type of drives used to store the data. Currently the following two classes are configured.

fast
The fast storage class is backed by NVMe storage and is the default for boot volumes created when launching an instance. Quota for this storage class is much smaller compared to the slow storage class below.
slow
The slow storage class is backed by normal 3.5" HDDs and is used for larger volumes. As storage on HDDs is available in bigger quantities the quota is significantly larger than that for the fast storage class.

Impact on project volume quota

As mentioned earlier in this documentation one might see an error message like the following. This occurs when trying to launch an instance with a larger boot volume than the default provided by the image. Another case for this error is when other instances are already running.

VolumeSizeExceedsAvailableQuota

Requested volume or snapshot exceeds allowed gigabytes_fast quota. Requested 120G, quota is 30G and 0G has been consumed.

The problem here is that the quota display on the overview page does not differentiate between the two storage classes. Instead, it only displays the combined volume quota which is the sum of the slow and fast quotas.

Unfortunately, there is currently now way to make this easier to understand in the dashboard. Therefore, keep in mind that the default quota for fast storage is only 80 GB while the slow storage quota is 240 GB resulting in an overall Quota of 320 GB.

Other issues

This might also limit the amount of instances that can be launched although the Instances quota itself has not been exceeded, depending on the source images used. Choosing a smaller source image would be a valid option to not run into this problem.

Possible resolutions

If you just require more storage for an instance it is advised to create a secondary volume using the slow storage class and attach the volume to the instance. More information on how to do that is available here.

If you still feel that you need more fast quota, contact the h_da cloud team to investigate your issue in detail and determine if there is a valid reason for increasing the fast storage quota.