How To
Summary
High Speed adapters need significant memory to buffer data to gain high performance.
Objective
Steps
N-Port Id Virtualisation (NPIV) and virtual Fibre Channel adapters - like high speed physical adapters (see an earlier blog) do require memory to operator at full speed. NPIV effectively turns the Virtual I/O Server (VIOS) into a kind of virtual SAN switch where packets are passed through with no changes from the physical adapter to the client Virtual Machine (LPAR). I found it very hard to find a recommendation in the documentation but was given a rule of thumb. As before it should be noted that starving the VIOS and the Hypervisor of memory is not a good idea - you may get away with it in small machine if your workloads do not make a high demand for performance but its better to budget in enough memory for these resources to peak high if the demand is there.
So we use what has become known as Austin's Rule of Thumb = 140 MB per virtual Fibre Channel adapter - this is additional Hypervisor memory but I am not sure who Austin is :-) This is memory you should allow for when sizing or designing a POWER machine as an administrator does not allocate memory to the Hypervisor - it just grabs what it needs.
With, say, just two physical adapters connected by NPIV to a small handful of client Virtual Machines you may have already configured enough memory in the machine. Most people are allowing in a sizing activity 2 to 4 GB of memory in a VIOS these days on larger machines, some in the VIOS itself and some as the Hypervisor memory.
Larger configurations of NPIV clients need to be calculated. For example: two VIOS supporting 20 client VMs, each client has one virtual Fibre Channel adapter on each VIOS = 40 virtual Fibre Channel adapters at 140 MB each = 5.6 GB memory. That might be a shock! But if you compare that to 40 physical Fibre Channel adapters, you will need 20GB of memory. So virtual is still a winner in just memory terms.
Of course, virtual also wins in
- less adapters (a large cost avoided),
- less adapter slots (avoiding a remote I/O drawer or two),
- increased flexibility (just add a new client VM when you need it),
- faster response to changing circumstances (no need to order H/W, delivery or install it) and
- you can pool physical adapters at the VIOS for higher bandwidth than low numbers of adapters per client VM.
It is a "no-brainer"really - How can any one justify not going pure virtual?
Comments Welcome.
Additional Information
Other places to find content from Nigel Griffiths IBM (retired)
- YouTube - YouTube Channel for Nigel Griffiths
- AIXpert Blog
Document Location
Worldwide
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Document Information
Modified date:
14 June 2023
UID
ibm11125417