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

[Jay Swackhamer]

Are you using powerpath or the qlogic multi-path, or no-multipath?
Are you using the default installed driver or the one that does multipath?

For qlogic.
Here's a utility to scan luns, you have to specify the max lun.
http://download.qlogic.com/ms/53595/readme_dynamic_lun_util_18.html

Are your ports zoned through switches or direct fibre from host->storage?

Quoting 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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-- 
Jay Swackhamer
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