System information

Array-Based (Third-Party) Solution
When you use an ESX/ESXi system in conjunction with a SAN, you must decide whether array-based tools are
more suitable for your particular situation.
When you consider an array-based solution, keep in mind the following points:
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Array-based solutions usually result in more comprehensive statistics. With RDM, data always takes the
same path, which results in easier performance management.
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Security is more transparent to the storage administrator when you use RDM and an array-based solution
because with RDM, virtual machines more closely resemble physical machines.
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If you use an array-based solution, physical compatibility RDMs are often used for the storage of virtual
machines. If you do not intend to use RDM, check the storage vendor documentation to see if operations
on LUNs with VMFS volumes are supported. If you use array operations on VMFS LUNs, carefully read
the section on resignaturing.
File-Based (VMFS) Solution
When you use an ESX/ESXi system in conjunction with a SAN, you must decide whether host-based tools are
more suitable for your particular situation.
When you consider a file-based solution that uses VMware tools and VMFS instead of the array tools, be aware
of the following points:
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Using VMware tools and VMFS is better for provisioning. One large LUN is allocated and multiple .vmdk
files can be placed on that LUN. With RDM, a new LUN is required for each virtual machine.
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Snapshotting is included with your ESX/ESXi host at no extra cost. The file-based solution is therefore
more cost-effective than the array-based solution.
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Using VMFS is easier for ESX/ESXi administrators.
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ESX/ESXi administrators who use the file-based solution are more independent from the SAN
administrator.
Managing Duplicate VMFS Datastores
When a LUN contains a VMFS datastore copy, you can mount the datastore with the existing signature or
assign a new signature.
Each VMFS datastore created in a LUN has a unique UUID that is stored in the file system superblock. When
the LUN is replicated or snapshotted, the resulting LUN copy is identical, byte-for-byte, with the original LUN.
As a result, if the original LUN contains a VMFS datastore with UUID X, the LUN copy appears to contain an
identical VMFS datastore, or a VMFS datastore copy, with exactly the same UUID X.
ESX/ESXi can determine whether a LUN contains the VMFS datastore copy, and either mount the datastore
copy with its original UUID or change the UUID, thus resignaturing the datastore.
Chapter 6 Managing ESX/ESXi Systems That Use SAN Storage
VMware, Inc. 73