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What limits the number of OS's?

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Donald Scott Nelson
Occasional Advisor

What limits the number of OS's?

What limits the number of OS's that can be installed on an array and accessed simultaneously by multiple hosts?
4 REPLIES
Brian M Rawlings
Honored Contributor
Solution

Re: What limits the number of OS's?

It depends on the array. In older arrays (and some current offerings), an FC port on an array must be set to a "port mode", which essentially means an OS mode for that port. Since most arrays are set up as fully redundant with path failover and dual connections to each host, this normally means that two FC ports are dedicated to one OS.

So... how many FC ports to hosts does the array have? If four, you are limited to two different OSes, one per pair of ports. If 8, you can have up to four OSes, etc. 8 FC ports is normally the max for any "modular array" (everything except the big monolithic frames, EMC Symmetrix or HDS Lightning; both of these can do up to 32/64 FC ports, and are practically SANs in one box -- and they run their ports different than the modular arrays anyhow).

This limitation was well known and kind of stupid -- design limitations built into first or second generation boxes which were "good enough for starters", and indeed lots were sold (most to people with all SUN or all Windows or all HP-UX, so multi-OS wasn't a problem). But the current generation of modular arrays pretty much all have come up with ways around this, so that the FC port is now not given a special mode, but some virtual entity further into the array gets to do all the "special" stuff associated with a given OS.

HDS calls it "host storage domains", and it is assigned on a per LUN basis (you can do up to 128 different OSes in any Thunder 95xx array, with only two or four or eight FC ports).

HP EVA doesn't have this port-based limitation, although I believe that the older HP/CPQ arrays did. This is not a problem for modern STK arrays either. I'm not sure on the EMC Clariions, which used to have this as an issue, but surely must have found a way around it by now (one hopes).

One important consideration used to be licensing for different OSes cost money per OS. HP changed that, and now you can download a kit for any host OS they support, they are free. It doesn't appear to cost anything from HDS either, although you have to buy enough of their array SW to enable the "host storage domains" (once enabled, it works for any/all). STK gives you the first one for free, and then (last I heard) charges per OS after the first. So, on some arrays, lots of OSes can get expensive. But that's not a technical issue, just financial.

If the OS is assigned per LUN, the max LUN count could limit you. Again, in modern arrays, this is such a large number, it is immaterial to the question. Some older or low-end arrays can only do 16 or 32 LUNs, but I don't deal with them, or rarely.

That's all I can think of, hope it helps.

--bmr
We must indeed all hang together, or, most assuredly, we shall all hang separately. (Benjamin Franklin)
Sanjay Kumar Suri
Honored Contributor

Re: What limits the number of OS's?

With regard to out expereince of using XP range of arrays, there is no such limit the number of OS's that can be installed on an array and accessed simultaneously by multiple hosts.

However there are limits on the number of hosts groups per CHIP port, number of total host groups per array, number of WWNs per host group.

sks
A rigid mind is very sure, but often wrong. A flexible mind is generally unsure, but often right.
Brian M Rawlings
Honored Contributor

Re: What limits the number of OS's?

Sanjay: the HP XP arrays are the HDS Lightning monolithic storage arrays that I mentioned earlier. HP and Sun both resell the Lightning line, which HDS calls the 9900 series (9970 ~ XP128; 9980 ~ XP1024).

And you're right, the big monolithic boxes are quite different in how their host ports function than the many modular arrays out there, including the HDS 95xx "Thunder" modular line.

In general, the Lightning arrays (like the big EMC Symmetrix units) can present all LUNs out of any and all ports simultaneously, with no ports limited to any given OS. Modular arrays, on the other hand, can only present LUNs out of the ports on one controller at a time, with the other controller as a hot standby in case of failure.

This concept of "LUN ownership" by one or the other controller is one of the major differences between modular storage and the big monolithic arrays. As I said before, however, it is rare these days to find that this also limits how many OSes can be serviced by one array or FC port, modular or monolithic.

Regards, --bmr
We must indeed all hang together, or, most assuredly, we shall all hang separately. (Benjamin Franklin)
Donald Scott Nelson
Occasional Advisor

Re: What limits the number of OS's?

All,

I wanted to thank everyone for their input, all information that everybody has provided has really helped me in getting a better grasp on this.

Thanks again,
Scott