Virtualization Oversubscription - What’s so scary? Oversubscription (3 of 20)

So what is oversubscription? In its simplest terms it’s allocating more than you have.

Thin Provisioning

The most obvious example of Thin provisioning happens in storage.  It’s been around for a long time and is the same thing simply known by another name.

With storage you have the LUNs that are allocated.  Now traditionally these would have been a physical allocation on disk that was available for use but with Thin Provisioning you can allocate more space to LUNs than you actually have.  The reason being, that most disks on servers are not full.  So if the average disk is 30% full, you could get away with only having 50% of your allocated storage as real usable space that exists, and you’d still have plenty of space to grow into.

On top of that, some storage systems will do their own deduplication. 

Deduplication & Compression

Imagine you have 200 Windows 2012 servers, all with a C drive that just has the OS on it.  That’s about 12 GB per server storing the same base OS files or 2.4TB of space.  Now those OS disks need space for things like memory dumps, updates and log files etc, which is the unused space but do you want to spend 2.4TB of storage storing the same 12GB of files 200 times?  Probably not.

 

You’d prefer to store a single copy of all the identical files and let them all access that single copy.  So you’re not just ignoring some of the unused space, you’re able to store less as well.

So, 200, 32 GB drives (Minimum Windows 2012 requirement), would be 6.4TB.  It’s now theoretically come down to something like 20GB used space with thin provisioning and deduplication.

On Wednesday I'll deal with what can be over-subscribed.

In the meantime our VMware online workshop is coming round fast so make sure you book your place

http://www.metron-athene.com/services/online-workshops/capacity-management-workshops.html

Phil Bell
Consultant