It is a little known fact that besides compute (capacity and performance), storage capacity and external network throughput rate, vCloud Director can also manage storage IOPS (input / output or read and write operations per second) performance at provisioned virtual disk granularity. This post summarizes the current capabilities.
Cloud providers usually offer different tiers of storage that is available to tenants for consumption. IOPS management helps them to differentiate these tiers and enforce the virtual disk performance based on IOPS metric. This eliminates noisy neighbor problem, but also makes both consumption and capacity management more predictive.
vCloud Director relies on vSphere to control the maximum IOPS a VM has access to on particular storage policy through a Storage I/O Control functionality which is supported on VMFS (block) and NFS datastores (no vSAN). In vSphere this is defined at virtual hard disk level, but is enforced at VM level. vSphere however does not manage available IOPS capacity of a datastore the same way it can do with compute. That’s where vCloud Director comes in.
The cloud provider first needs to create a new vSphere custom field (iopsCapacity) and use it do define for vCloud Director managed datastore their IOPS capacity. This is done via vCenter Managed Browser Object UI and is described in KB 2148300.
vCloud Director consumes vSphere datastores through storage policies. In my case I have tag based storage policy named: 2_IOPS/GB and as the name suggests the intention is to provide two provisioned IOPS per each GB of capacity. 40 GB hard disk thus should provide 80 IOPS.
Once the storage policy is synced with vCloud Director we can add it to a Provider VDC and consume it in its Org VDCs. vCloud Director will keep track of the storage policy IOPS capacity and how much has been allocated. That information is available with vCloud API when retrieving the Provider VDC storage profile representation:
Note that the pvdcStorageProfile IopsCapacity is the total IopsCapacity for all datastores as tagged in vCenter belonging to the storage policy.
The actual definition of storage policy parameters is done via PUT call at Org VDC level again with API on the Org VDC storage profile representation. The cloud provider supplies IopsSetting element that consists of the following parameters:
- Enabled: True if this storage profile is IOPS-based placement enabled.
- DiskIopsMax: the max IOPS that can be given to any disk (value 0 means unlimited)
- DiskIopsDefault: the default IOPS given to any/all disks associated with this VdcStorageProfile if user doesn’t specify one
- StorageProfileIopsLimit: the max IOPS that can be used by this VdcStorageProfile. In other words: maximum IOPS that can be assigned across all disks associated with this VdcStorageProfile (use 0 for unlimited).
- DiskIopsPerGbMax: similar to DiskIopsMax but instead of a specific value, it’s the ratio of size (in GB) to IOPS. if set to 1, then a 1 GB disk is limited to 1 IOPS, if set to 10, then a 1 GB disk is limited to 10 IOPS, etc.
When a user deploys a VM utilizing IOPS enabled storage policy she can set specific requested IOPS for each disk though API (0 is treated as unlimited), or set nothing and vCloud Director will set default limit based on DiskIopsDefault or DiskIopsPerGbMax x DiskSizeInGb value, whichever is lower. The requested value must always be smaller than DiskIopsMax and also smaller than DiskIopsPerGbMax x DiskSizeInGb. The DiskIopsMax and DiskIopsDefault values must also be lower that StorageProfileIopsLimit.
In my case I wanted always to set IOPS limit to 2 IOPS per GB, so I configured Org VDC storage policy in the following way:
And this is provisioned VM as seen in vCloud Director UI
and in vCenter UI.
- Datastore clusters cannot be used together with IOPS storage policies. The reason is that when datastore clusters are used it is vCenter who is responsible for placing the disk to a specific datastore and as mentioned above, vCenter does not track IOPS capacity at datastore level, whereas the vCloud Director placement engine will take into account both the datastore capacity (GB) and IOPS capacity when finding the suitable datastore for a disk.
- vSAN is not supported as it does not support SIOC. vSAN advanced storage policies allow specifying IOPS limits per object and can be used instead.
- Disk IOPS can be assigned only to regular VMs, not to VM templates.
- The disk IOPS will be always allocated against the Org VDC storage profile even if the VM is powered-off. This means the cloud provider can oversubscribe IOPS at the provider VDC storage profile level.
- System administrator can override IOPS limits when deploying/editing tenant VMs in the system context.
- Some vCloud Director versions have bug where the UI sends 0 (unlimited) IOPS for disk instead of null (undefined) which might result in provisioning error if it is not compliant with the policy limit.