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Concurrent access and you

A definition and some unresolved issues about concurrent access.

Concurrent access and you
Christopher Poelker

This tip comes from one of our Ask the Expert forums. Christopher Poelker, the expert answering this question, is a storage architect at Hitachi Data Systems.

Q: What is concurrent access?

A: As it pertains to storage, it is the ability to access a particular LUN from either two different paths on a single host simultaneously (load balanced), or by two separate hosts within a cluster environment (distributed lock management).

The problem to overcome is when two hosts try to "change" the data at the same time. This is where a lock manager is needed to allow cooperation between the hosts for data access. Within a storage array, using dual ported fibre drives, the firmware can load balance a drive over both ports, and "queue" I/O across the "least full" path. This is a form of "concurrent access" also. Things get real interesting when you use a "universal" file system. By universal, I mean a file system that is SAN based and is understood by all operating systems. This is very similar to file based access protocols like NFS and CIFS, but is "block" based for high performance applications. There are many vendors working on this type of solution, but it will need the involvement of ALL the OS vendors. This will mean cooperation between SUN (Solaris), Microsoft (NT, Win2K), IBM(AIX), HP(HP-UX), Compaq (VMS, True64 Unix), and all the Linux variants. Something this broad will need to be driven by a standards body like SNIA, FCIA, IETF or the IEEE.

Additional Resources:
1. How are SCSI target LUNs mapped to Fibre Channel LUNs?
This reader submitted the following question to our storage networking expert Marc Farley: "Can you fully explain how SCSI target LUNs are mapped to Fibre Channel LUNs with persistent binding (across boots) on Fibre Channel switched networks, SAN for AIX, SUN, and NT systems?"

2. What are the pros and cons of zoning a SAN?
While zoning a SAN provides a number of advantages in storage administration and security, there are several ways to do it with different benefits and drawbacks. In addition to zoning at the device level or LUN level, there is also hard, soft and persistent zoning. This tips introduces the various zoning approaches.

3. How can I evaluate various storage solutions?
Given that there are so many emerging technologies in SAN and NAS, it is really getting difficult for the users to choose the right product and technology. This user asked our SAN expert Christopher Poelker, "What is the performance basis a user should see while evaluating different storage systems?" Chris came back with an awesome checklist.

4. How can I manage growth with aggregated NAS appliances?
Find out how to put real muscle behind managing storage growth! This searchStorage Real-Time Vendor Connection with Tricord Systems and storage networking expert Marc Farley focused on the benefits of aggregation in a NAS environment.

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