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SCS Standard Flavors and Properties



In OpenStack environments there is a need to define different flavors for instances. The flavors are pre-defined by the operator, the customer can not change these. OpenStack providers thus typically offer a large selection of flavors.

While flavors can be discovered (openstack flavor list), it is helpful for users (DevOps teams), to have a guaranteed set of flavors available on all SCS clouds, so these need not be discovered.

Properties (extra specs)

The following extra specs are recognized, together with the respective semantics:

  • scs:name-vN=NAME (where N is 1 or 2, and NAME is some string) means that the flavor is one of the standard SCS flavors, and the requirements of Section "Standard SCS flavors" below apply.
  • scs:cpu-type=shared-core means that at least 20% of a core in >99% of the time, measured over the course of one month (1% is 7,2 h/month). The cpu-type=shared-core corresponds to the V cpu modifier in the flavor-naming spec, other options are crowded-core (L), dedicated-thread (T) and dedicated-core (C).
  • scs:diskN-type=ssd (where N is a non-negative integer, usually 0) means that the root disk N must support 1000 sequential IOPS per VM, and it must be equipped with power-loss protection; see scs-0110-v1-ssd-flavors. The diskN-type=ssd setting corresponds to the s disk modifier, other options are nvme (p), hdd (h) and network (n). Only flavors without disk and those with diskN-type=network can be expected to support live-migration.

Whenever ANY of these are present on ANY flavor, the corresponding semantics must be satisfied.

Standard SCS flavors

Following are flavors that must exist on standard SCS clouds (x86-64). Note that this statement does not preclude the existence of additional flavors.


Recommended namevCPUsvCPU typeRAM [GiB]Root disk [GB]Disk type
Recommended namevCPUsvCPU typeRAM [GiB]Root disk [GB]Disk type

Guarantees and properties

The figures given in the table (number of CPUs, amount of RAM, root disk size) must match precisely the corresponding figures in the flavor.

In addition, the following properties must be set (in the extra_specs):

  • scs:name-v1 to the recommended name, but with each dash AFTER the first one replaced by a colon,
  • scs:name-v2 to the recommended name,
  • scs:cpu-type to shared-core or crowded-core, reflecting the vCPU type,
  • scs:disk0-type not set if no disk is provided, otherwise set to ssd or some other value, reflecting the disk type.


We expect the most used vCPU

ratio to be 1:4.

Note that all vCPUs of SCS standard flavors are oversubscribed — the smallest 1L-1 flavor allows for heavy oversubscription (note the L), and thus can be offered very cheaply — imagine jump hosts ...

The design allows for small clouds (with CPUs with 16 Threads, 64GiB RAM compute hosts) to offer all flavors.

Except for the two flavors with SSD root volume, disks types are not specified (and expected to be network disks (Ceph/Cinder) or local SATA/SAS disks typically).

We only included a limited variation of disk sizes — this reflects that for the standard networked cinder disks, you can pass block_device_mapping_v2 on server (VM) creation to allocate a boot disk of any size you desire. We have scaled the few recommended disk sizes with the amount of RAM. For each flavor there is also one without a pre-attached disk — these are meant to be used to boot from a volume (either created beforehand or allocated on-the-fly with block_device_mapping_v2, e.g. openstack server create --flavor SCS-1V-2 --block-device-mapping sda=IMGUUID:image:12:true to create a bootable 12G cinder volume from image IMGUUID that gets tied to the VM instance life cycle.)

Conformance Tests

The script will read the lists of mandatory and recommended flavors from a yaml file provided as command-line argument, connect to an OpenStack installation, and check whether the flavors are present and their extra specs are correct. Missing flavors will be reported on various logging channels: error for mandatory, info for recommended flavors. Incorrect extra specs will be reported as error in any case. The return code will be non-zero if the test could not be performed or if any error was reported.

Operational tooling

The openstack-flavor-manager is able to create all standard, mandatory SCS flavors for you. It takes input that can be generated by

Previous standard versions

The list of standard flavors used to be part of the flavor naming standard up until version 3. The following changes have been made to the list in comparison with said standard:

  • the flavor names have been turned into recommendations, and
  • the properties have been introduced in order to help discoverability.

Note that the flavors with fixed size root disks have all moved to Recommended with scs-0100-v3. This means that they are not a certification requirement any longer, but we still recommend implementing these for backwards compatibility reasons. Also in that standard, two flavors with SSD+ root disks have been added, as defined in