[olug] MAX_LUNS: Caveat about selling Linux for SAN to "the management"

[swanpoint at cox.net]

We have an BL45p (HP AMD blade) connected to an EMC Symmetrix DMX 2000p (if all that matters). RedHat w/ linux-2.6.9-42.0.3.ELsmp. Emulex cards w/ driver version 8.0.16.27. Problem manifested also w/ QLogic cards.

With Symmetrix there is a lot of disks too be partitioned into a whole lot of LUN. We're using 4 GB and 8 GB sizes for our LUN's; so the numbers increment quickly. There can be 256+ LUN's behind a FA port (on the Symmetrix end). Not all LUN's are mapped to the same server; but, if the LUN index is greater than 255, no good.

Doing a quick and dirty "find . -type f -exec grep -i max_luns {} \;" inside the linux-2.6.19 kernel source gives me the impression that the SAN drivers (Emulex and QLogic) can had more than 255 LUN.

There's a kernel parameter for specifying the max number of lun, but that parameter has no effect (recommended solution for RedHat support is to specify the max lun parameter at boot, too no avail).

---- Christopher Cashell <topher-olug at zyp.org> wrote: 
> At Wed, 17 Jan 07, Unidentified Flying Banana swanpoint, said:
> > If you plan to implement Symmetrix, CLARiiON, iSCSI, or any other tech 
> > that could make use of LUN greater than 255; be aware that Linux won't 
> > recognize a LUN higher than 255 (but AIX, HPUX, and Windows do). Be sure 
> > you don't go out and buy a ton of DL585's, DL380's, Dell 1955's, HP 
> > BL45p's, and stacks of QLogic and Emulex cards and hope it works cuz' it 
> > don't...
> 
> I can't help but think this sounds a little bit over the top and like
> the knee-jerk response of frustration.  Are you actually dealing with a
> situation where a 255 LUN limit is causing you problems?  I've been
> using Linux with various SANs for quite a while, including some fairly
> decent sized setups, and it's never been an issue.
> 
> The issue I've had a lot more trouble with is running out of raw
> devices, specifically on Oracle RAC configurations using pre-2.6.x
> kernels (max of 255 raw devices).  Honestly, I'm having trouble coming
> up with much in the way of realistic situations where I would anticipate
> a problem with the LUN limit, at least outside of very rare and unusual
> configurations.
> 
> Can you detail your setup and where exactly the problem is coming in?
> 
> -- 
> | Christopher
> +------------------------------------------------+
> | Here I stand.  I can do no other.              |
> +------------------------------------------------+
> 
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