The global reserved LUN pool works with replication software, such as SnapView, SAN Copy, and MirrorView/A to store data or information required to complete a replication task. The reserved LUN pool consists of one or more private LUNs. The LUN becomes private when you add it to the reserved LUN pool. Since the LUNs in the reserved LUN pool are private LUNs, they cannot belong to storage groups and a server cannot perform I/O to them.

Before starting a replication task, the reserved LUN pool must contain at least one LUN for each source LUN that will participate in the task. You can add any available LUNs to the reserved LUN pool. Each storage system manages its own LUN pool space and assigns a separate reserved LUN (or multiple LUNs) to each source LUN.
All replication software that uses the reserved LUN pool shares the resources of the reserved LUN pool. For example, if you are running an incremental SAN Copy session on a LUN and a snapshot session on another LUN, the reserved LUN pool must contain at least two LUNs - one for each source LUN. If both sessions are running on the same source LUN, the sessions will share a reserved LUN.

Estimating a suitable reserved LUN pool size

Each reserved LUN can vary in size. However, using the same size for each LUN in the pool is easier to manage because the LUNs are assigned without regard to size; that is, the first available free LUN in the global reserved LUN pool is assigned. Since you cannot control which reserved LUNs are being used for a particular replication session, EMC recommends that you use a standard size for all reserved LUNs. The size of these LUNs are dependent on If you want to optimize space utilization, the recommendation would be to create many small reserved LUNs, which allows for sessions requiring minimal reserved LUN space to use one or a few reserved LUNs, and sessions requiring more reserved LUN space to use multiple reserved LUNs. On the other hand, if you want to optimize the total number of source LUNs, the recommendation would be to create many large reserved LUNs, so that even those sessions which require more reserved LUN space only consume a single reserved LUN.

The following considerations should assist in estimating a suitable reserved LUN pool size for the storage system.
If you wish to optimize space utilization , use the size of the smallest source LUN as the basis of your calculations. If you wish to optimize the total number of source LUNs , use the size of the largest source LUN as the basis of your calculations. If you have a standard online transaction processing configuration (OLTP), use reserved LUNs sized at 10-20%. This tends to be an appropriate size to accommodate the copy-on-first-write activity.
If you plan on if you plan on creating multiple sessions per source LUN, anticipate a large number of writes to the source LUN, or anticipate a long duration time for the session, you may also need to allocate additional reserved LUNs. With any of these cases, you should increase the calculation accordingly. For instance, if you plan to have 4 concurrent sessions running for a given source LUN, you might want to increase the estimated size by 4 – raising the typical size to 40-80%.

5 Comments:

  1. Root Says:
  2. Is the limitation of eight concurrent sessions for a given LUN still the case for a CX3-80? And is this limitation a hardware limitation (i.e. the next CLARiiON, assuming beefier hardware, would increase that limitation?) or a software limitation (the increase would come with another FLARE code release).

    Thanks!

  3. Diwakar Says:
  4. Yes, This is software based limitation as far as size concern for reserved LUN Pool. If you want have 8 session then increase the estimated size by 8.

    Thanks,
    Diwakar

  5. Root Says:
  6. Wait, I thought the max amount of concurrent snap sessions is 8. You make it sound like you can have more?

  7. Diwakar Says:
  8. You are right, you can have 8 session on each LUN.

    In Brief:

    -- Eight snapshot element per source.
    -- 100 Source(Depend on Model) per storage system
    -- 800 session/300 Snapshots(Depend on Model) per storage system.

    Thanks,
    Diwakar

  9. sudhir b Says:
  10. how many max number of luns can be added to reserved lun pool

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