Re: [PLUG] SAN Questions

[Jeff McAdams <jeffm@iglou.com>]

Sean Cummins wrote:
> - You'll find supported HBA's available from Emulex or Qlogic.
> - HBA drivers are generally not included in the Linux distributions --
> you will have to load them post-install.  Or if you're using VMware
> ESX server, the drivers are included in the VMware distro.

FWIW, it has been my experience that distros (as long as they're halfway
modern) do include common HBA drivers as many of the drivers are
included in the mainstream Linux kernel.  Now, they may not be the
versions of the drivers that are supported by the SAN storage vendors,
but basic HBA drivers are there for at least the common HBA types.  We
use Qlogic, FWIW.

> - You'll need the HBA drivers themselves, HBA firmware, management
> utilities (eg. SANSurfer or HBAnyware), array management host software
> (eg., Solutions Enabler, Navisphere), and multipath software (eg.,
> Powerpath).

<opinion type=personal>Just say "NO" to proprietary multipathing
software.</opinion>

Linux mainstream kernels, and thus most of the modern distros support
standard (Linux device mapper multipathing), and standard'ish (Qlogic
driver level multipathing) capabilities for multipathing.

Our experience was with HP SecurePath and HP EVA's, and we had *severe*
problems with SecurePath.  When we converted over to Qlogic's
multipathing capabilities, *all* of our issues with multi-pathing went
away (to HP's credit, this is the direction they're going for their
support of Linux).  I would like to move to device mapper based
multi-pathing, but we haven't done the engineering work yet to try that
out and make that happen.

> - The HBA drivers are supported by the HBA manufacturers and EMC.
> - Powerpath has essentially the same look, feel, and functionality on
> Linux as Solaris/HPUX/AIX.  VMware ESX has its own multipathing built
> in.
> - I'm not aware of any hot-swappable HBA support on Linux.

I have no personal experience on this last point either.  I'm not sure
how well Linux PCI support handles hot-swap PCI in general, HBA's would
essentially be a special case of that.

> - There are several ways to discover new SAN devices from Linux.. the
> easiest would be to either reboot, run the rescan utility included
> with the Emulex driver, or reload the HBA driver.

We use utilities that are provided by HP to rescan the bus dynamically
and pick up new LUNs presented to the machines.  Those utilities are
merely shell scripts, though, so its quite possible to do this in a
vendor-agnostic manner, I just haven't dug into the scripts to see what
exactly they're doing for sure to figure out how they're doing it.
Shouldn't be hard, it certainly is possible. without reloading the
driver or rebooting the machine.

> - Once the new devices are visible to the host, you can use
> essentially the same procedures for creating filesystems as you would
> with a local disk.  eg., use fdisk to create a partition, then mkfs to
> create a filesystem on the partition.  You'll want to create your
> partition on the Powerpath pseudo device (assuming you're using
> Powerpath), and you'll want to align the partition to the underlying
> LUN during creation, for optimal performance.

Yeah, once the LUNs are presented to the host and are visible, you get a
/dev/sd? device that shows up.  If you're using udev or the like, you
can configure it to create symlinks for you that give you stable naming
for your LUNs, even if they get probed in different orders each time.
Or, you can use volume labels, or you can use LVM or LVM2 and let the
device mapper system figure out what goes where for you, or ..., or ....

There are a number of different options for handling that, depending on
your policy, procedures, sysadmin styles, whether you prefer the color
green, or the number 12.
-- 
Jeff McAdams
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor safety."
                                       -- Benjamin Franklin

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