- Is it possible to do the clone right through the server? I realize that
there's dhcp configuration. When can I use it?
This depends on how you are using it. At its heart, in my opinion, udpcast is like a big network pipe that works one-to-many. What you do with the pipe is up to you.
You could use it to clone a complete disk I suppose, but that would mean you'd want the clone source and clone target to be the same hardware nearly exactly I'd assume -- both due to the drivers in your initial ram disks and due to the need for the image to fit block-per-block without exceeding space. I think people have used it this way, but unless you have multiple disks or partitions, you still need some sort of environment to host the udp-receiver on the client side whether that means putting it in to an initrd or using something else to fill that role.
What has typically done for cases where you use udpcast as a image transfer engine is you integrate it with some sort of cluster manager. Systemimager integrates it as a component in systemimager-server-flamethrowerd. Flametrhower manages the udp-sender processes on the admin node and maps content to instances of the server. Through it's install environment, the clients boot in to the systemimager initrd/environment where a udp-receiver is installed. If you boot the nodes at the same time, they can all join the same stream for the disk image. In the cases of systemimager I've seen, they would typically stage the image as a tarball on the node in TMPFS, then extract it after the transfer. This works better because the transfer can be done to memory, reducing retries (hard disks can induce retries). I believe this is configurable of course. In this use case, the image is typically a tar file created on the fly through the pipeline. However, there is still pre and post-configuration to get this going. Pre-configuration might be making disk partitions and filesystems and post-configuration may be things like ensuring the newly installed nodes install bootloaders and configure themselves.
In my area, we use udpcast in a case similar to systemimager although we have our own environment that replaces the systemimager function. As I mentioned, we're still using it with current distros and now even with the aarch64 architecture. I prefer the udpcast stream approach to other multicast approaches that use file block lists. This is because, whenever possible, I want to avoid having a image file that is a copy of an image directory layout. It is painful to have to maintain files that represent directory layouts, push them around to multiple administrative servers in a big node, and re-create and re-push all that stuff each time the master image is changed. So with udpcast, the tar is created and piped on-demand. We even integrated encryption in to the stream using openssl in some cases where customers have wanted that.
On really big systems, we have multiple administrative servers (leader nodes) that all have udp-sender instances managed by flamethrower currently, We tend to handle 288 node per leader in this mechanism although we've had higher ratios. udp-sender currently supports up to 1024.
One challenge area with udpcast in my opinion is multicast support in switches -- especially smart switches -- have made pain points for us in large clusters. It can be painful to get groups of switches from different vendors to behave well with multicast in some cases. We've even had to resort of "flood all ports" in some desperate scenarios. In some cases where the management switches had been decided ahead of time by other teams, we've had to switch to using bittorrent instead of udpcast to avoid multicast issues. Bittorrent means you need a file instead of a stream, so there is added maintenance in making sure the tarball represents the current image copy and is the same on all the admin servers. Still, with everything properly configured on a clean network, it's hard to beat the magic of having 10,000 nodes all installing at once through their respective administrative (leader) nodes and VLANs using udpcast.
Best wishes,
Erik