Ask the Expert

Creating and allocating storage as needed

Should all LUNs on a SAN be the same size? We're building a new SAN with Brocade switches, W2K, AIX, Solaris and Linux servers and multiple RAID units based on CMD Titan controllers. We're faced with a choice of carving up all the storage up front so that all LUNs are the same size or building LUNs as we go along and making them whatever size is needed. What is recommended best practice in this situation?

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I would sit down and plan everything out upfront as far as what types of RAID are needed for which environments. Use a mixture of RAID5, RAID1 and RAID0+1 based on the needs of the applications on each OS.

Once you have your RAID types planned out, you will be able to figure out how much storage space will be lost to parity information. This will give you your base useable capacity. Next you will need to determine how much more space is wasted after a file system is applied to your LUNs. This will give you your true useable capacity. Once everything is planned out and you're sure you have enough room for all your data, then you can plan how to partition your disks for allocating capacity. If your storage arrays are limited in cache size, then it would be best practice to lay out your storage so that each application uses it own separate physical disks. This will be easy to do with the multiple arrays your talking about. Spread the I/O load around as much as possible.

All that being said, and all your plans in place, if your storage array will let you create LUNs and partition them on the fly, then allocate the storage down the road as needed. This is because even the best laid out plans have a tendency, in our business, to change overnight due to some unforeseen application needing to be added. Creating and allocating storage as needed, will give you greater flexibility down the road. Of course, if you need to reboot the array every time a RAIDSET is added and formatted, then do it all up front.

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This was first published in January 2003

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