Q

Storage networking vs. bus-attached storage

I'm new to SAN and this might be answered somewhere else but I haven't found it.

There are a lot of issues as to why storage networking is better than bus-attached storage. From the performance perspective, I think there is nothing to gain. At least not until there are disks that can be directly attached to the Fiber Channel. But if you look at the disk subsystem at the end of the Fiber Channel cable, it has SCSI-attached disks. It seems to me that I achieve nothing. I end up with SCSI disks, which I can easily have in the server. Even worse is the added complexity and performance penalty since the data must travel an extra path through Fibre.

Am I missing something here? Of course I'm looking from the performance perspective, not the management of a large amount of disks.

Here are the things you missed:

  1. Most storage array vendors are moving to Fibre disks on the back end, and a few like HDS, Clarion and MTI have them now.
  2. Fibre provides DISTANCE to the disks at 100MB per second.
  3. Using two fibre HBAs enables fault tolerance, and can provide load balancing at 200MB per second to the disks!
  4. Fibre just moved to 2Gbit, offering a dual HBA host to have 400MB per second access to the disks.
  5. SAN arrays are SMART and can offer data replication and snapshots in hardware for disaster recovery and zero backup windows.
  6. SAN arrays have a lot of CACHE which can offer much better performance than direct-attached disks.
  7. Clustering requires storage centralization and is a no-brainer using a SAN.
  8. A SAN can provide MASSIVE amounts of storage to be seen by a host over a single adapter.
  9. You can stick up to eight Fibre HBAs in a host all accessing a SINGLE LUN, for 1600MB per second access if using 2Gb HBAs.

Those are just a few of the benefits. Adding storage to a pool in a SAN is non-disruptive to the host, so you may never have to bring your servers down again. SANs can also do dynamic LUN expansion if you are running out of room on a disk. A single person can manage Terabytes of SAN storage, saving "people" revenue for your company. SAN arrays are also more dependable, keeping your operation running more effectively with less downtime.

From a performance perspective, the only direct attached storage I have seen that beats SAN storage is a solid state disk, but some SAN vendors allow you to create a "solid state disk" from array cache.

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This was first published in September 2001

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