What you will learn: Expanding and partitioning LUNs is explained.

Some storage systems support partitions to help allocate unique LUNs to different servers. Depending on implementation,

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some vendors support partitions that incorporate LUNs from different volume or RAID groups for flexibility, or if a LUN can span multiple storage enclosures and I/O busses inside the storage system to avoid performance contention.

Another feature that will vary by vendor implementation is how LUN expansion takes place if supported. For example, vendors will often support on-line growth of RAID or volume groups to facilitate creation of additional LUNs, while others support dynamic expansion of existing LUNs. However, do not assume that all implementations are the same and that dynamic expansion means while data is being read or written to a LUN. Check the details and ask vendor-specific questions as to what it does or does not support and if there are any extra fees for enhanced LUN functionality.

In addition to the number of LUNs supported, the size of LUNs supported will also vary by storage system and operating systems. Most storage devices and operating systems today should be able to support at least a 2 TB LUN. Some operating systems can support addressing and accessing of LUNs larger than 2 TB either natively or via upgrade patch kits.

For storage systems, look at the total number of LUNs supported by a particular solution or if that vendor has guidelines or restrictions on the number of LUNs that can be accessed on a given physical or adapter port. Also, check to see what limitations on addressing or flexibility exists with volume mapping and masking software features in storage systems to enable specific LUNs to be seen or hidden from different servers.

For LUNs being accessed by nonclustered or failover server nodes, typically you would hide those LUNs from other servers so that they are not seen and accessed. Similarly on the server, path management software from vendors, such as Microsoft's MPIO, IBM's SDD, Symantec Corp.'s DMP and EMC Corp.'s Powerpath, should be configured to access and support path failover to LUNs accordingly.

About the author: Greg Schulz is founder and senior analyst with the IT infrastructure analyst and consulting firm StorageIO Group. Greg is also the author of Resilient Storage Networks (Elsevier) and a contributor to Storage magazine and other TechTarget venues.


This was first published in January 2008

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