Friday, January 28, 2011

File Server - One large LUN or multiple smaller LUNs?

We're about to replace a bunch of kit (servers, SAN etc.) and migrate our servers across from their current platform.

Our file server has around 8tb of data on it right now, spread across 5 LUNs (all on the same SAN mind you).

To my mind the pro's of multiple LUNs mainly come down to "what if" things, such as file restores, impact of file system corruption, and the con's are allocation of space i.e. I might have 500gb free on one LUN but no space free on the LUN where I need it.

How would you do it and why?


  • I dont see a real reason to split it over multiple LUN.

    From TomTom
    1. Check whether there are any limitations on max LUN size on your SAN or at any other point in your stack. For example, if you're running this as a VM within ESX then you have a limit of 2Tb per LUN without hacking around it.

    2. Inspect the payloads. If you're serving several distinct kinds of content from the same server (e.g. Install ISOs, User Shares, Email Archives) you may want some of them to be on faster disks while some sit on cheaper disks. Could this tiering be of value in the future even if it's not required today?

    3. Evaluate the likely future growth of your storage requirements. Is it likely that you'll ever have to increase the available space within one of your storage areas? Can you do this while the LUN stays online? Can you do this while the server stays online? You may find that by splitting your data logically across several LUNs, you can expand one of those storage areas while serving from just 1 server, and without down-time.

    4. Weigh up the probability of ever needing to split one of your storage areas off to a separate server. Performing this operation is simpler if that storage is on a separate LUN, as you can de-provision it to the current server and provision it to the new host.

    5. Are you replicating your storage to another device? Is this on a per-LUN basis? Do you need to replicate all of the data, or only part of it? You may want to split into seperate LUNs to cut your replication traffic down to just the important data.

    Hutch : SAN shouldn't care (P4000), LUNs may be RDM in which case yes 2tb limit applies, or they may be presented within the guest from the MS iSCSI initiator. I think point 2 is very relevant as with a single large LUN it does seem I give myself fewer options.


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