[TriLUG] SAN file locking

[Greg Cox]

On Fri, Dec 16, 2011 at 7:42 PM, Joseph Mack NA3T <jmack at wm7d.net> wrote:
> I haven't setup a SAN so don't know how people handle file locking.
> I assume the SAN file locking problem is the same as for other shared file
> systems eg. for multiple hosts mounting disks via NFS.

When you need some storage, you usually want NAS.
When you need a disk, you usually want SAN.

> Is everyone just mounting /usr (ro) and sharing that, while mounting home
> directories (rw) expecting the user to handle the exclusive writing
> manually?

That, IMO, would be a bad use of SAN, from either direction.  Let's
say you have 10 machines, and your SAN is FC-based.

1) OK, /usr COULD be a shared r/o LUN, but.. boring.  You'd have your
LUN visible r/o to everyone, but.. how did you boot?
It's a crying shame to spin local disk AND pay for 10 HBA's.  Why not
just simplify your life and make 10 LUNs, say, 20GB each, and put the
whole OS on it?  Map then 1-to-1 to the farm, and boot that 'disk'.
If your storage controller is worth its salt, you spent 20GB just then
because the blocks are going to be thin-provisioned.  They COULD
un-deduplicate and cost you the whole 200GB in time, but, still, 200GB
in your controller is still nicer than the 20 spinning drives (you
WERE going to mirror them, right?)

2) Why would you put /home on a LUN?  What happens when you have a
gaming rig and want to get your /home files on there via SMB... but
you have an ext3-formatted LUN?  So, you want storage, not necessarily
a 'disk'.   Make a NAS share and have autofs mount it up when you log
in on Linux/Solaris, and SMB on Windows?  Oh, sure, NFS locking is
nobody's friend, but, if you're running something more than vi against
your homedir, you're probably not being a good architect.

> In this case there is no real sharing. If there's no real sharing,
> what are SANs being used for?

Boot LUNs.  Swap LUNs for stupid Oracle boxes that require more swap
than they'll ever use.  Backing storage for old versions of Oracle
that refused to do locking over NFS.  ESX datastores.