Pool LUN overhead

Then please explain why I had 7GB available in a pool and after I cteated a 1GB LUN I only had 3GB left and I wasn’t able to create another 1GB LUN.

First of all, if you make a 100GB LUN, you want to assign 100GB of space to a server. It would be very confusing if the VNX would just grab the overhead from that 100GB LUN, leaving you with a 97GB LUN. It would make sizing more of a trial and error process...

The actual situation is real simple. You create a lun, say 100GB. On top comes xGB of metadata + 2% of extra overhead. That's it!

Graphically, you'd see:

This is a 1024GB / 1TB LUN from server perspective, but on the actual pool it consumes a bit more. The overhead is just the Consumed Capacity - User Capacity.

But that's simple RRR:

Start 7GB free in the pool.

I create a 1GB LUN. In reality, it uses 1GB for user space, ~3GB for overhead.. Total usage = 4GB.

Pool free: 7-4=3.

If i would want to create another 1GB LUN, i'd need again 1GB of user space, 3GB of overhead. Which is 4GB, which doesn't fit in a pool that only has 3GB free. Error!

That’s a lot of overhead, percentage wise that is.

Yeah, but let's be honest... how often do you allocate 1GB LUNs? I think 9 out of 10 LUNs are 1TB over here, for the VMware farm. The rest are some physical servers that use some storage; LUN sizes range from 10 to 600GB.

Almost never, but in this config I had to create 25TB worth of LUNs and I left the 1GB LUNs for last. Don’t ask me what they were for, could be quorum disks, I don’t remember.

Van: Jon Klaus

Verzonden: donderdag 13 september 2012 15:15

Aan: Rob Koper

Onderwerp: New message: "Pool LUN overhead"

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Re: Pool LUN overhead

created by Jon Klaus in VNX Support Forum - View the full discussion

Jon, thank you.

I understand the differences betwen user capacity and consumed capacity. But i can`t understand why does a very small lun has so huge amount of metadata. It is not a problem for me, and usualy i don`t care about 3 GB loss. But I`m interested in so-called "deep technical knowledge"   EMC`s white papers do not give a definition what exactly those 3 GB contains. They say just "metadata"

Same here. I did not see this coming.

VNX 1 use 1GB slices. So that is 1024 slices to to track and store in metadata for a 1TiB LUN.

3% is the overhead for LUN creation.

When I create a 1TiB LUN (1024GiB) I get an overhead of 31GiB. 3.4% on 1024 so pretty close.

Maybe the smallest overhead it can use is 3GiB?

Correct PacmanSAN, there's always a minimum of 3GB of overhead.