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More questions and answers from SAN School lesson #3

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SAN School: Lesson 3



"What makes a SAN go?

In this third lesson of SAN School, you learned about the inside mechanics of a SAN. Lesson 3, "What makes a SAN go?" covered latency, or moving data over a distance, and bandwidth, which is the process of moving the data through your network or bandwidth. You also learned about Fibre Channel, SCSI and iSCSI networking protocols as well as the fabric name server and how to perform zoning. Chris also explained the different types of zoning such as port zoning, world wide name zoning, and mixed zoning which are designed to help you allocate resources on your SAN. Chapter three concluded with a discussion of logical unit numbers and how data is assigned to a server.

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:

** Different methods of failover, log shipping vs. replication
** How an SATA drive hooks up to a SAN
** Explanations of port zoning, aliases and name servers
** The difference between a Volume and Logical Unit Number (LUN)
** Vendor recommendations for HBAs, cables, GBICs, and switches
** LUN Zoning
** What happens when SAS and SATA HDs hit the market
** Masking and what it is
** Why port zoning is more

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secure than WWN zoning

Back to the SAN School table of contents

Question: In a SAN environment, what method of failover is most recommended, log shipping vs. replication?

Professor Poelker: In a SAN, the term "failover" is usually used to indicate a dual attached host's access port failover to another HBA. Host port failover is accomplished with a "filter driver" located either in the OS (Like MPXIO in Solaris, or MPIO in Windows), or via a driver provided by the storage vendor (Like Powerpath from EMC, Securepath from Compaq, or HDLM from Hitachi).

In the case of your question, I take the term "failover" to mean application level failover to another location, which would imply a form of data replication. There is no basic recommended solution, since the solution chosen would require an assessment of your environment. If you have the bandwidth between sites, and your distance between sites is short enough to keep latency low, then SYNC replication would be the way to go. If there is a great distance between sites, then ASYNC replication would be better. If you have limited bandwidth, then log shipping would be the next method I would choose.

Question: How can an SATA drive hook up to a SAN?

Professor Poelker: Storage array manufacturers are building arrays that can house either SCSI drives, Fibre Channel drives, ATA drives, or SATA drives within the array. The controllers front ending the connection between the drives and the SAN fabric bridge the protocols to allow connectivity.

Question: You made a comment that port zoning doesn't use the Name Server. It does. What did you mean by this? You comment that the name server will complain when merging a fabric if you use non-unique alias names. Aliases are not stored in the name server. What does the comment mean?

Professor Poelker: My comment was geared toward the differences between "HARD" zoning and "SOFT" zoning. By saying port zoning does not use the name server, I meant to indicate that port zoning is enforced via the hardware, and that when using hard or "port" zoning, frames not destined to the zoned ports are barred by the hardware from those ports. Soft zoning, or "WWN" zoning uses only software (the name server) to enforce the zones (this is changing with newer switches) and "frames are not barred from being transmitted between nodes that are not in the same zone" (quoted from building SANS with Brocade, by Chris Beauchamp).

The alias server is a type of "name server" that uses extended link service requests (Alias_ID) to refer to multiple N_ports through a single name. My comment was to warn users who attempt to integrate separate fabrics with zoning in place that they will run into difficulty connecting the fabrics together if they have used the same alias names for anything.

Question: What is the difference between a Volume and Logical Unit Number (LUN)?

Professor Poelker: A volume normally refers to a "disk" created via a "volume manager" such as Veritas, or a volume created by an operating system, such as Windows NT. A LUN refers to a "logical unit number" presented to a host as a SCSI ID. (i.e., LUN number 1 specifies SCSI ID 1 on that port. Therefore, the term volume can be considered software based, and LUN considered hardware based).

Question: Which vendor products do you recommend for HBAs, cables, GBICs, and switches?

Professor Poelker: I do not make specific recommendations, since I try to always be vendor agnostic.

Question: You discuss zoning methods. How does LUN zoning come into play with the methods you discuss?

Professor Poelker: LUN zoning comes into play as a second level of security on top of fabric based zoning. LUN zoning is normally done at the storage array level, by admitting only specific fabric WWNs to access the LUN within the array. This is also known as "LUN masking". LUN masking, used in conjunction with fabric based zoning, provides the best security for your SAN.

Question: What do you think will happen when SAS and SATA HD's hit the market - which one WINS? Where and Why?

ATA drives are already being shipped within storage arrays as a low cost alternative for applications like data archiving and regulatory compliance. SATA drives are also now appearing. SATA drives have a faster interface than normal ATA disks, and can have higher duty cycles. SATA will be used where more performance than ATA disks is needed, but cost is still an issue. You will see both types as a replacement for tape within virtual tape arrays used for disk class backup, and as an archive repository.

Question: What is Masking? Is it sort of the reverse of zoning, where you say a port or WWN cannot talk to another specified WWN or port?

Professor Poelker: Masking is actually "LUN masking", and is used to provide security for LUN access at the storage array level. It is not the reverse of zoning. It is used in conjunction with zoning to provide two levels of security in the SAN.

Question: On the port zoning and mixed zoning slides, what does the line connecting Port 0 on Switch 2 and Port 4 on Switch 1 represent?

Professor Poelker: It is not labeled, and does not appear on the WWN Zoning slide. That was not a line it was just part of the larger circle that represents zone 2. If you look at the graphic closer, you will see that the only link between the two switches is the ISL link between ports 1 and 5.

Question: Chris said that of the two types of zoning, WWN and port, that port zoning was more secure. I'd like to know why he thinks port zoning is more secure than WWN zoning.

Professor Poelker: I learned most of the things I know about zoning by playing with the older Brocade 2800 switches. Newer switches offer better security when zoning using WWN or physical ports. On the Brocade 2800, WWN zoning is accomplished in software, and port zoning is done at the hardware ASIC level. When using hard or "port" zoning, frames not destined to the zoned ports are barred by the hardware from those ports. Soft zoning, or "WWN" zoning uses only software (the name server) to enforce the zones (this is changing with newer switches) and frames are not barred from being transmitted between nodes that are not in the same zone.

If you missed lesson three 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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