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Full Discussion: what is SAN
Top Forums UNIX for Dummies Questions & Answers what is SAN Post 52899 by keelba on Tuesday 29th of June 2004 12:08:23 PM
Old 06-29-2004
I've always believed that there is a bit of confusion in terms that people use. I have always thought of NAS as Network Area STORAGE and SAN as Storage Area NETWORK. Anything that connects to this special network is a NAS device and it runs on a SAN. That SAN can be ethernet or fibre channel. However, I think that the common usage for most people is that SAN means fibre channel network and NAS means ethernet.

So by that definition a SAN is a network of fibre channel (a protocol) connected by either copper wires or fiber optic cables. Each device on this SAN connects through an HBA (hardware bus adapter) and 1 or more fibre channel switches. When the SAN is first installed every device on that SAN can see every other device such as a server, a tape drive, a disk drive, a tape library, disk array, etc. The SAN administrator will then set up paths to allow certain devices to "see" only certain devices and block paths to other devices. It is possible to have more than 1 computer using a certain device. This is sometimes used for clustering failover or pehaps a shared tape drive.

When you boot your computer it will see every device on the SAN that it has access to and it will look like a locally attached disk to your computer. There is no security for these devices other than that path. For example, if both a unix server and an NT server see the same drive the NT system will not know about the unix system and will claim it as its own and write its own header on that drive destroying the data for the unix system. Many people will refer to their disk array as their SAN but in reality the SAN is the network connecting the disk array. You may call the array a SAN device but not a SAN itself.

I may be slightly off on some of this and please correct me if I am wrong but this is always how I've thought of it.
 

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st_san_safe(5)							     OBSOLETED							    st_san_safe(5)

NAME
st_san_safe - determine whether access to tape device special files without the no-rewind-on-close option is allowed (OBSOLETED) VALUES
Failsafe Default Allowed values Recommended values or DESCRIPTION
Note: this tunable is obsoleted and has been renamed to beginning with release HP-UX 11i v3. This tunable notifies the tape driver whether it should allow access to device special files without the no-rewind-on-close option. When this tunable is on the tape driver will fail any open on a tape device special file without the no-rewind-on-close option (that is, or with a status of The tunable allows the safe sharing of tape devices in SAN tape backup solution configurations. The disabling of device special files without no-rewind-on-close in a SAN tape backup solution configuration prevents the accidental sending of a rewind command on close from HP-UX systems via standard tape commands like which would corrupt a backup in process. Who Is Expected to Change This Tunable? Anyone. Restrictions on Changing Changes to this tunable take effect immediately. When Should the Tunable Be Turned On? This tunable should be turned on if the system is going to be used in a SAN tape backup solution configuration. What Are the Side Effects of Turning the Tunable On? The tape driver will fail any open to a tape device special file that does not have the no-rewind-on-close option with a status of This behavior prevents the usage of any tape device special file without the no-rewind-on-close option. When Should the Tunable Be Turned Off? This tunable should be turned off if the system is not part of a SAN tape backup solution configuration or the system utilizes will not function unless it has access to device special files without the no-rewind-on-close option. What Are the Side Effects of Turning the Tunable Off? A rewind may be issued from this system accidentally on close possibly corrupting a backup being issued from another system on the SAN. What Other Tunables Should Be Changed at the Same Time? None. WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX. Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tun- able values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see at AUTHOR
was developed by Hewlett-Packard. SEE ALSO
norewind_close_disabled(5), scsi_tape(7), scsimgr_estape(7). Tunable Kernel Parameters st_san_safe(5)
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