As I understand SVmotion.

1. ESX creates a snapshot of the VM.
2. ESX starts copying the base vmdk file to the new storage as well as the config files.
3. ESX consolidates the snapshot file on the old storage into the base vmdk on the new storage.
4. When complete, it transfers control to the new storage.
5. Clean up the old storage location.

This way if there is an interuption at any point, all of the files still exist in the old location and it fails back without disruption.

So the only writing that is happening across the storage is the copy process and consolidation process.

Charles Killmer, VCP

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The copy process goes over whatever connection exists between the host and the storage. Some SAN integrations allow for ESX telling the SAN where the files exist and having the SAN perform the copy without the networks involvement. This makes the copy process very fast. Not many vendors provide that yet though. If you do not have this integration, and it sounds as though you do not, then it will copy across the iSCSI/FC/NFS network and be subject to bottlenecks.

Charles Killmer, VCP

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