Rackspace requires that customers have a certain percentage of each Cluster Shared Volume (CSV) datastore free at all times. We call this datastore overhead. Use the following guidelines to ensure that there is enough datastore overhead:
- 10% free space, if the partition size is less than 1TB
- 10% free space, if the partition size is between 1TB and 5TB
- 10% free space, if the partition size is greater than 5TB
Why does Rackspace require this?#
If a datastore runs out of free space, it causes any VMs running on that datastore to immediately hard stop, causing downtime. Stopped VMs cannot be started until insufficient free space on the datastore us available.
After VMs are stopped, it is possible that a VM might not power on due to insufficient space on the datastore.
VM performance impact is possible when the datastore is critically low on space, and certain Hyper-V features, such as checkpoints, might no longer function.
What else can consume space on my datastores?#
- Checkpoints: When performing a checkpoint, a delta disk is created for each disk you have on your VM. While the checkpoint is active or a export is in progress, all changes are written to the delta file(s). Delta files can grow rapidly and without your awareness.
- VM log files: All VM log files are stored within the VM container folder on the datastore. Normal log files are only a few MB in size. However, log files can grow larger in size depending on VM activity.
- RAM: Powered on VMs have a VM BIN file equivalent in size to the
amount of RAM assigned to the VM.
If a VM’s
AutomaticStopActionis set to
Save, Hyper-V reserves disk space equal to the amount of memory used by the VM when it is running. This ensures that memory can be written to disk when the physical computer shuts down. If the VM is powered off, there is no BIN file present, even if the VM is set to save.