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Chapter 14. Working With Storage
140
When a snapshot is taken manually, a snapshot is always created regardless of whether a volume has
been active or not.
14.5.4. Snapshot Restore
There are two paths to restoring snapshots. Users can create a volume from the snapshot. The
volume can then be mounted to a VM and files recovered as needed. Alternatively, a template may
be created from the snapshot of a root disk. The user can then boot a VM from this template to effect
recovery of the root disk.
14.5.5. Snapshot Job Throttling
When a snapshot of a virtual machine is requested, the snapshot job runs on the same host where
the VM is running or, in the case of a stopped VM, the host where it ran last. If many snapshots
are requested for VMs on a single host, this can lead to problems with too many snapshot jobs
overwhelming the resources of the host.
To address this situation, the cloud's root administrator can throttle how many snapshot jobs are
executed simultaneously on the hosts in the cloud by using the new global configuration setting
concurrent.snapshots.threshold.perhost. By using this setting, the administrator can better ensure
that snapshot jobs do not time out and hypervisor hosts do not experience performance issues due to
hosts being overloaded with too many snapshot requests.
Set concurrent.snapshots.threshold.perhost to a value that represents a best guess about how
many snapshot jobs the hypervisor hosts can execute at one time, given the current resources of the
hosts and the number of VMs running on the hosts. If a given host has more snapshot requests, the
additional requests are placed in a waiting queue. No new snapshot jobs will start until the number of
currently executing snapshot jobs falls below the configured limit.
The admin can also set job.expire.minutes to place a maximum on how long a snapshot request will
wait in the queue. If this limit is reached, the snapshot request fails and returns an error message.
14.5.6. VMware Volume Snapshot Performance
When you take a snapshot of a data or root volume on VMware, CloudPlatform uses an efficient
storage technique to improve performance.
A snapshot is not immediately exported from vCenter to a mounted NFS share and packaged into
an OVA file format. This operation would consume time and resources. Instead, the original file
formats (e.g., VMDK) provided by vCenter are retained. An OVA file will only be created as needed, on
demand. To generate the OVA, CloudPlatform uses information in a properties file (*.ova.meta) which
it stored along with the original snapshot data.
Note
For upgrading customers: This process applies only to newly created snapshots after upgrade
to CloudPlatform 4.2. Snapshots that have already been taken and stored in OVA format will
continue to exist in that format, and will continue to work as expected.