More questions and answers from SAN School lesson #2

By SearchStorage.com editors 13 Jan 2004 | SearchStorage.com

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SAN School: Lesson 2

In this second lesson of SAN School you heard an in-depth discussion on the issues associated with building a SAN. You also got some advice on how to make your SAN scalable so you can get the greatest cost benefits for both the short and long term. SAN School Lesson 2 also explained the difference between modular and monolithic storage arrays, and the different types of RAID and the benefits of each.

Since so many of you asked Chris Poelker questions during this SAN school Webcast, he didn't have time to answer them all. Therefore, we sent those questions to Professor Poelker and are posting them along with the answers for you here.

Here you'll find answers to questions pertaining to:

Back to the SAN School table of contents

Question: How does parity work? Is it data or information about the data? How can data be "recovered" from parity?

Professor Poelker: Parity works by using a mathematical formula (usually an exclusive OR operation) on the data before it is stored on the disks. RAID (redundant array of independent disks) works by storing the data in chunks across all the drives in the "raid set." The parity information (the result of the mathematical formula applied to the data) is stored separately from the data itself (usually striped across all the drives in the raid set). If one of the drives goes bad, and you loose the "chunks" of real data stored on that drive, then the RAID controller re-creates the original information by using the formula in reverse, to calculate the original data that was written. You could say that parity is data about the data, or "metadata", but that term does not usually apply to parity.

Question: What is the difference between LUNs and Volumes?

Question: Great Lesson! I have also heard of RAID levels 3 and 4. Could you explain these?

Professor Poelker: Under RAID levels 3 and 4, the parity data is stored on a single dedicated disk, rather than being "rotated" across all the disks in the raid set. If your application needs to access large blocks of sequentially addressed data, then RAID 3 or 4 may be a better method.

Question: If the SAN is being used to enable a clustered database between a fixed small number of nodes (say <6), what is the best cost/performance solution?

Professor Poelker: The best solution for this environment would be a "hub" based SAN using the FC-AL (Fibre Channel arbitrated loop) protocol. A generic SCSI cluster would make sense for a two node cluster. Over two nodes, SCSI gets cumbersome. FC-AL components have become quite cheap, and a simple FC-AL based Fibre Channel shelf with disks installed will be much cheaper than a raid array.

Professor Poelker: In Brocade switches, there is a technology used called "Quickloop" that enables a bridge between FC-AL devices on the port that is considered a quickloop, and the rest of the devices in the fabric. Fabric based devices can reach FC-AL destination addresses on the loop, since the switch associates a fabric address to the FC-AL addresses within the loop.

All switches use FC-SW as the native protocol. The ability to connect to legacy FC-AL devices through a switch that supports FC-SW to FC-AL address translation allows you to re-use older hub (FC-AL) based devices like tape drives.

Question: What is the name of the 1st mirror disks and strips on top of them (it gives more felexibility at the disk failure time)?

What you are suggesting is called RAID 1+0, or RAID10 (raid ten) for short. It is always better to mirror disks first, and then do striping, rather than create a mirrored stripe set. If you stripe first, and then mirror, and you lose a drive, the entire stripe set is disabled. If you mirror two disks at a time, and then stripe, you can survive multiple disk failures.

Question: How difficult is it to move from a modular array to an enterprise array as needs require you to make the transition?

The latest versions of storage management solutions are moving ahead with a way to classify data via policy, and then use a method of HSM (hierarchical storage management), which uses the software itself, or a function of the hardware to automatically migrate data between platform classes based on the created policy. This allows you to create an SLA (service level agreement) on specific data types or applications, add the application to the policy, and the data ends up on the correct storage based on the SLA. In the meantime, you can always use the host OS to create mirrors of your volumes (one member on the modular, and the other on the monolithic) to migrate data between platforms. Your storage vendor can also help you do this via a services agreement when you want to make the move.

Question: Does every frame get converted from digital to optical through the switch?

Professor Poelker: Yes, that is the function of the GBIC (gigabit interface converter) on every switch port. The light pulses coming into the switch are converted to digital data, the switch looks into each frame to find the destination address, routes the data to the correct switch port that is attached to the target, and the GBIC on the target port converts the signal back into light pulses for re-transmission to the target. This all happens extremely fast. Within the next ten years, I expect to see optical switches on the market that will eliminate that requirement. Optical or "photonic" switches will be able to move terabytes of data per second.

If you missed lesson two of SAN School, view it anytime here

About Christopher Poelker:

Aside from being an author and a SearchStorage.com SAN expert Christopher Poelker is a storage architect at Hitachi Data Systems. Prior to Hitachi, Chris was a lead storage architect/senior systems architect for Compaq Computer, Inc., in New York. While at Compaq, Chris built the sales/service engagement model for Compaq StorageWorks, and trained most of the company's VAR's, Channel's and Compaq ES/PS contacts on StorageWorks. Chris' certifications include: MCSE, MCT (Microsoft Trainer), MASE (Compaq Master ASE Storage Architect), and A+ certified (PC Technician).

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