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When you create a disk image, you must choose where it is stored, under the heading Datastore.
You have the choice between local_images_ssd and ceph.
When a VM is created, images on the datastore local_images_ssd are copied to the node where the VM will run.
This takes some copying at startup and, if the image's persistent=yes, copying back at shutdown, but during the run of the VM the data is local.
Always place the (small) boot disk with the OS on datastore local_images_ssd.
Other disks may be on datastore ceph, depending on your needs.
Large data images are problematic, as the disk space needs to be available on the node where the VM runs.
When a VM is created, images with persistent=yes on the datastore ceph are directly connected as a network disk to the node where the VM will run.
This takes no copying at startup and all data written to the VM's disk will go over the network to the image.
This implies that if the VM is deleted or crashes, the image may be corrupt, just as with a real hard disk connected to a PC.
Large data images are possible, as no disk space is used on the node where the VM runs.
When a VM is created, images with persistent=no from the datastore ceph are copied for the private use by the VM.
This takes some copying at startup and the copy is directly connected as a network disk to the node where the VM will run.
Large data images are possible, as no disk space is used on the node where the VM runs.