Hi Mark,

We are trying to do some R&D on SAN and I have some doubts. Suppose we have two machines (A & B). One, (A) has an FC hard disk and the other, (B) has a SCSI disk and iSCSI is running between these two m/cs. Can we communicate between the two hard disks? To make it happen what are the functions we should implement on the machines?

Thanks in advance.


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The communication between a machine and its storage occurs at what I call the "storing" level. This is typically a protocol such as SCSI or some form of serial SCSI, such as FCP for FC networks or iSCSI in IP networks. This is NOT a system-to-system communication vehicle. That said, FC can carry IP as an optional protocol for system-to-system communications and iSCSI sits on top of TCP/IP that is the most common system-to-system protocol. Still if you are thinking about communicating with disk drives, then you should segregate the traffic in your mind to keep things clear.

The system function that oversees these exchanges is at the "filing" level. Two machines can communicate on a filing level using NAS technology (NFS or CIFS) protocols. This is not a case where one machine directly accesses the hard disk of another machine, but makes the request of the other machine's file system - this is typically referred to as remote mounting of a file system. There is no direct protocol between the remote (requesting) system and the hard disk where the data is.

There is ONE file system that controls the allocation of space on the disk. Two systems do not share this role. Therefore in traditional systems, there is no way to share the disks between systems.

However, there are SAN file systems today using FC that allow multiple systems to share disks. This requires the use of a third system that acts as the access control for the other machines. IBM, HP, Compaq and others sell these solutions. This type of file system is not available under iSCSI today but there is no reason why it wouldn't work someday in the future.

Regards,
Marc Farley


This was first published in July 2001

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