Q

How to determine the number of HBAs per server

What variables should I use when trying to determine the number of HBAs I need on a server?

A reader recently asked this question:

I am trying to determine the number of HBAs I need on a server. I would like to know what variables I need to use.

Some of the variables I can think of are:

1) amount of storage
2) I/O rate
3) I/O block size
4) RW ratio
5) speed
6) sustained utilization of HBA

Do you have any rules of thumb?

Chris Poelker responded:

Yeah, the basic rule of thumb for how many HBAs to use is two for disk, and one for tape. Using 2 GB fibre channel, across two fabrics, two 2 GB storage ports, you should be able to stream data at over 300 MB per second. If you're worried about IOPS, then two connections should be fine for most data intensive applications. The only time I have needed to use more than two adapters on a particular server, is in massive digital media environments and humongous data warehouses.

I always specify another adapter for tape streams if the server itself is involved in pushing data to tape. If not, and you are using serverless backup through the SAN, then two adapters should be fine. Each adapter should be able to handle between 128 and 256 LUN numbers, depending on the SAN environment. If your connecting a Windows host, you will run out of drive letters WAY before you run out of LUNS.

Chris


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This was first published in November 2003

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