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Why does MCS take a snapshot only to flatten it asap. Why not clone directly?


george zabetas

Question

I am racking my brain trying to figure out why a snapshot is created vs a clone.
Everything I am reading points to a clone, in that you have a full backup of the image, as to not be tampered with.

Appears MCS creates a snapshot of the VM, then creates a flat clone and makes copies of that for the VMs.
Wny not go directly to the clone.
If fallback is needed, they can fallback to the original VM which is still intact no?

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To summarize my confusion in my research.
This is what happens with MCS.
1. VM created with its disk.
2. Shapshot is taken and a differencing disk is added.

3. Snapshot is flattened and no more differencing disk .   (huh?)
4. New differencing disks created for the cloned VMs. (double huh?)

This seesaw behaviour of adding and removing difference disks is maddening.


And to top it off, Explanation given is that so you can fall back on the snapshot.

Here is the caper. The shapshot is taken after the apps are installed on your image. So  to fall back to what? The apps are already on there.
Second point of confusion.
Seems the snapshots are taken after app installs.

My common sense would dictate this recipe.

1. Create your VM.
2. Clone your VM (for your protection)

3, Install apps.
4. Distribute new VM

If problems use your clone from step 2.

WOuld appreciate some light into this matter.
Thanks

 

 

Edited by gzabetas
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3 answers to this question

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The whole idea of MCS is that it falls back onto the hypervisor snapshot technology of choice - be it vSphere, HyperV, XenServer, Azure, AWS, GCP, its a full handoff back to hypervisor. This obviously comes with some overheads and annoyances, but it's been pretty solid

 

Snapshots are always taken after app installs, when they refer back to rolling back to snapshots, they mean prior snaps - you can have as many (depending on hypervisor and performance) snaps associated with a master VM as you want, allowing for full rollback and forward as required. The process from an MCS standpoint takes the snapshot, creates a clone of it to all storage repositories that will host VM workloads (The multi LUN tax....) and then spins up VM's based on that snapshot clone, should you need to go back, it falls back to the copy of the snap it had on the storage repo prior, or if it's gone, it repeats the process (there are some cleanup tasks that run)

 

You will see different behaviours associated with diff disks depending on if you have done thin vs full clones etc

 

You probably know half of that already, so if you need more detail i can try and get some good eyes on this post from internal at Citrix

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Thanks James,

Ok if rolling back to snapshots refer to some assumed earlier snapshots we take, what is the point of taking a snapshot after the app installs? They only serve to create clones, Cut the middle man and just go to the clone?

 


What is the purpose of this second snapshot if its only to create a clone and we will never fall back to it,  if anything we would fall forward (lol) since its a post app snapshot.

Is it possible that the clone that spins the VMs gets a virus and then maybe this 2nd snapshot is used for a REFRESH scenario.

I cant see why we wouldnt go directly to the clone.
Unless hypervisors cant do that. I read somewhere that even a clone creates a temp snapshot in its process.  Could that be the missing piece.
I hate how all training materials brushes over this.

 

 

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Not sure on if the hypervisor side of things has issues with clones etc, I just know from an mcs standpoint we are snapshot happy - we regularly have environments with multiple catalogs pointing to different snapshots in a chain (test and uat etc) and roll forward and rollback through snaps is very common in troubleshooting scenarios  

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