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

[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.              |
> >> +------------------------------------------------+
> >>
> >> _______________________________________________
> >> OLUG mailing list
> >> OLUG at olug.org
> >> http://lists.olug.org/mailman/listinfo/olug
> >
> > _______________________________________________
> > OLUG mailing list
> > OLUG at olug.org
> > http://lists.olug.org/mailman/listinfo/olug
> >
> 
> 
> 
> -- 
> 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.
> 
> _______________________________________________
> OLUG mailing list
> OLUG at olug.org
> http://lists.olug.org/mailman/listinfo/olug