Data Storage - Newbie: SAN access

Author

Newbie: SAN access

manuel

2005-08-18, 5:48 pm

Hello,

As far as I understand, when you have a SAN network with a fibre switch
and several computers want to access to the same resource (hard disk
arrays) a file-locking software is necessary. If it is not used, every
computer sees the resource as private. If I am not correct, please, let
me know.
These file-locking softwares, are part of the operating systems
(windows, OS X, linux) or must they be got from third parties? Does
anybody know any of these softwares?
Is the big picture similar for iSCSI SANs? Is there any tool for the
initiators share the resource or is this funcionality built in the
initiators?

Thanks,
Manuel

robertwessel2@yahoo.com

2005-08-18, 5:48 pm

manuel wrote:
> As far as I understand, when you have a SAN network with a fibre switch
> and several computers want to access to the same resource (hard disk
> arrays) a file-locking software is necessary. If it is not used, every
> computer sees the resource as private. If I am not correct, please, let
> me know.
> These file-locking softwares, are part of the operating systems
> (windows, OS X, linux) or must they be got from third parties? Does
> anybody know any of these softwares?
> Is the big picture similar for iSCSI SANs? Is there any tool for the
> initiators share the resource or is this funcionality built in the
> initiators?

First, you don't generally access a drive array on a SAN, you access a
volume (commonly called a LUN). The SAN storage box will partition the
disk storage (quite possibly an array) into as many volumes as you
want. So any sharing happens at the volume (LUN) level.

Most systems connected to a SAN just see volumes that are not shared
with anyone else, and it works just like a locally attached disk.

Two or more hosts can share access to a volume, but they must both know
how to do that. It's not trivial and the required support must be
built into the filesystem code in the OS's on the sharing hosts. There
are some minimal provisions for locks at the volume (LUN) level. One
host may "reserve" (aka lock) the device, and any other host will be
prevented from acquiring that lock before the first host does a
"release." This is supported in straight SCSI, Fibre Channel, iSCSI,
and a number of other I/O protocols. It's possible to use just device
reserve to implement shared storage (for example, you can do a lock
when updating any filesystem metadata), but most systems that allow
shared volumes need a communications channel in addition to the shared
disk to coordinate.

For non-disk devices on SANs, such as tape drives, simple device
reserve can be sufficient - if one host system wants to use the tape
drive, it can just reserve it, and then use it exclusively until it's
done.

boatgeek

2005-08-24, 8:47 pm

to elaborate a little, Windows 2000 cluster servers use something
called a cluster disk driver, which is built into the operating system
of advanced server and very simple to setup a basic share. The
cluster disk driver filters out the disk from the node which doesn't
own the resource. The nodes determine through a quorum whether the
operating system is still writing to the data device and it has a keep
alive pulse between the servers to determine the clusters network
status. The main limitation of windows cluster servers is that you
couldn't expand disks or add luns on the fly, but had to take down the
cluster servers.

A netware server does something similar in that it has a piece of the
operating system which also assigns ownership of a resource to a
particular device. Netware doesn't sync it's registry as do the
windows servers and give a poison pill a server which is having
problems. It can add luns on the fly, as can unix and linux.

there's really a lot more too this, a lot more. but if you're simply
seeking to share a lun, you're probably looking at a simple operating
system such as windows in a cluster. you can expand this to
geocluster geographically disperse nodes (though with windows they need
to be on the same vlan), go to very robust strategies of offsite
replication of LUNs for DR using virtual servers and vmware, but a
simple configuration is a fibre switch, a shared disk device and a
couple servers with HBAs.