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

[Jay Swackhamer]

Did you just try and specify the max_luns on boot or did you add
options scsi_mod max_luns=512   to modprobe.conf and rebuild initrd?

Quoting swanpoint at cox.net:

> Reason for the small LUN are historical --- "because it's always   
> been done that way here." It also fits "the workflow process" ---   
> users get a database, say 32 GB to start, and then ask for   
> incremental increases as data grows (in multiples of 4 GB).
>
> We're using the multipath(8) that shipped with RedHat. Ports are   
> zoned through switches.
>
> ---- Jay Swackhamer <Jay at RebootTheUser.com> wrote:
>> 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
>> Reboot The User
>> 13416 A Street
>> Omaha, NE 68144
>> 402-933-6449
>> http://www.reboottheuser.com
>> Hours: 6pm-11pm Tues-Fri, 4pm-9pm Sat.
>>
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-- 
Jay Swackhamer
Reboot The User
13416 A Street
Omaha, NE 68144
402-933-6449
http://www.reboottheuser.com
Hours: 6pm-11pm Tues-Fri, 4pm-9pm Sat.