Implementing LUNs
What you will learn: Greg Schulz explains how to create and implement LUNs.

LUNs can be created and implemented in various configurations, such as capacity, address, or boot device, along

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with what host servers can see or access the LUN or volume. Creation of a LUN can be done using vendor supplied management tools or third-party tools. LUNs can be created on storage systems and virtualisation appliances along with other storage devices, including virtual tape libraries (VTL). Some third-party storage management tools enable you to create LUNs on different vendor's storage systems, while other tools simply provide tracking and storage resource management (SRM) information for LUNs.

There are a few generic steps for creating a LUN. First, it is necessary to create a RAID group or volume group on the storage system or virtualisation appliance mapping some number of unused HDDs or devices together. The next step is assigning a RAID level and then subdividing or "carving" out LUNs from the RAID group assigning manually or automatically address numbers.

The number of LUNs supported by a storage system will vary by vendor implementation and storage I/O interface, such as Fibre Channel, SAS, Parallel SCSI, iSCSI, etc., being used. Likewise, various operating systems and virtual servers, including VMware, will have different limits on how many LUNs can be accessed in total and per adapter port on the physical server. Some operating systems also have a default or preference that certain storage, including boot devices, is on certain targets and LUNs. For example, older versions of Windows NT would not recognise nonzero LUNs until upgraded to newer versions and registry entry adjustments.

Some storage systems support the notion of partitions where a group of LUNs or volume groups can be mapped into a partition that appears to the host servers as a unique storage system with indicial LUNs. For example, 20 servers could be attached to an LSI/Engenio-based storage system that has four Fibre Channel ports. Assuming that the storage system has 40 total LUNs (or more), 20 partitions could be created with each partition containing two LUNs with one of the LUNs being LUN 0 as a boot device and mapped to a particular server adapter address. Each server would think that it has its own unique LUN 0 for a boot device yet in reality, there are 20 LUN 0s, each isolated from the other in a shared storage environment.

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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