Re: Newbie: SAN access

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.

.

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