This article was initially published by Mayur Vadhar in August, 2015:
Before we dive into the enhancements brought in the Storage XenMotion(SXM) feature in the XenServer Dundee Alpha 2 release; here is the refresher of various VM migrations supported in XenServer and how users can leverage them for different use cases.
XenMotion refers to the live migration of a VM(VM's disk residing on a shared storage) from one host to another within a pool with minimal downtime. Hence, with XenMotion we are moving the VM without touching its disks. E.g. XenMotion feature is very helpful during host and pool maintenance and is used with XenServer features such as Work Load Balancing(WLB)and Rolling Pool Upgrade(RPU) where the VM's residing on shared storage can be moved to other host within a pool.
Storage XenMotion is the marketing name given to two distinct XenServer features, live storage migration and VDI move. Both features refer to the movement of a VM's disk (vdi) from one storage repository to another. Live storage migration also includes the movement of the running VM from one host to another host. In the initial state, the VM’s disk can reside either on local storage of the host or shared storage. It can then be motioned to either local storage of another host or shared storage of a pool (or standalone host). The following classifications exist for SXM:
• When source and destination hosts happen to be part of the same pool we refer to it as IntraPool SXM. You can choose to migrate the VM's vdis to local storage of the destination host or another shared storage of the pool. E.g. Leverage it when you need to live migrate VMs residing on a slave host to the master of a pool.
• When source and destination hosts happen to be part of different pools we refer to it as CrossPool SXM. VM migration between two standalone hosts can also be regarded as CrossPool SXM. You can choose to migrate the VM's vdis to local storage of the destination host or shared storage of the destination pool. E.g. I often leverage CrossPool SXM when I need to migrate VM’s residing on pool having old version of XenServer to the pool having latest XenServer
• When the source and destination hosts are the same, we refer to it as LiveVDI Move. E.g. Leverage LiveVDI move when you want to upgrade your storage arrays but you don’t want to move the VMs to some another host. In such cases, you can LiveVDI move VM's vdi say from the shared storage (which is planned to be upgraded) to another shared storage in a pool without taking down your VM’s.
When SXM was first rolled out in XenServer 6.1 ,there were some restrictions on VMs before they can be motioned such as maximum number of snapshots a VM can have while undergoing SXM, VM having checkpoints cannot be motioned, the VM has to be running (otherwise it’s not live migration) etc. For the XenServer DundeeAlpha 2 release the XenServer team has removed some of those constraints .Thus below is the list of enhancements brought to SXM.
1. VM can be motioned regardless of its power status. Therefore I can successfully migrate a suspended VM or move a halted VM within a pool (intra pool) or across pools (cross pool)
2. VM can have more than one snapshot and checkpoint during SXM operation. Thus VM can have a mix of snapshots and checkpoints and it can still be successfully motioned.
3. A Halted VM can be copied from say pool A to pool B (cross-pool copy) .Note that VM’s that are not in halted state cannot be cross pool copied.
4. User created templates can also be copied from say pool A to pool B (cross-pool copy).Note that system templates cannot be cross pool copied.
Well this is not the end of the SXM improvements! Stay tuned with XenServer where in the upcoming release we aim to reduce VM downtime further during migration operations, and do download the Dundee preview and try it out yourself.
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