I'd like some opinions on the below, TIA:
Now every storage vendor has limitations. So lets keep this all in perspective shall we, everything is relative.
When you create a LUN using Netapp storage, you have the option to enable space reservations on the LUN. This is of course to ensure writes to the LUN should the volume ever fill up due to snapshots. So the general consensus is to set your volume size to 2.2-2.5 times the size of your desired LUN. So lets say you wish to create a LUN 100GB in size. To be on the safe side you would need to create a volume that is 250GB in size. Now only 100GB of that is actually usable space to the host.
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Now here is a moment where we get to keep everything relative. Ask 5 different people what “usable space” is and you will get 5 different answers. You are, “using” the space. You have a space efficient, near instantaneous backup and restore points. One could also say that because of this feature, it is “useful” to the host as well. Well just for this case lets say that it is not.
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So I have now ensured writes to my LUN BUT now have 150GB of space I can not use. So what is the solution? Disable space reservations on the LUN. In consequence what have a I lost? I can still take snapshots but if my snapreserve fills up the writes will fail and the LUN will go offline. So we can combat this by enabling snap-autodelete. Well that’s fine and dandy but lets consider a worst case scenario for a moment. Lets say I have a host connected to this non-space reserved LUN. Host gets a virus and observes 100% changes in my LUN. I am unaware of this virus and have had it going on for a week while I was on vacation. Snapshot schedule comes around and takes a snap. Oh we had a 100% change, not enough room for a snapshot, lets drop one. This happens to be the last non-virus infected snapshot. I get back from vacation and discover my host is hosed. So like any regular Netapp admin I go and create LUN clone or use flexclone to clone the LUNs from the snapshots to decide which one I want. I discover NO GOOD SNAPSHOT!!! Not good.
So now lets say we have the same scenario but this time I do not have snap-autodelete turned on. Well the writes just fail and the LUN goes offline. Everyone in the office hates me but at least I am able to get back to working state.
Another option could be to autogrow the volume. However, now you have something you need to manage much more closely. If using de-dupe you have to ensure at least 3% of available space in the aggregate.
Which do you choose?
Another thing to think about, is that this scenario of keeping snapshots, choosing to keep volumes online or using snap-autodelete is not just true of LUNs. As a matter of fact it’s a law of physics. This same scenario applies to CIFS and NFS as well. To truly be able to protect your data in an online fashion such as snapshots you must reserve 100% of space somewhere in the volume.
P.S. I am in no way saying that Netapp storage is bad. Far from it actually. I LOVE NETAPP!!! If you could guarantee me that they will never go out of business or do ANYTHING unethical I would get their logo tattooed on me Polynesian style! Some other storage vendors don’t even let you disable space reservations for LUNs. To me Netapp is the most flexible and agile storage vendor out there. What I want are strategies and best practices. Ways to tell my customer, “I know you are not happy about the space reserves but this is why you need to do it, or these are the risks you take if you don’t, this is how the Netapp experts are recommending you do it, etc.”