Creating parity groups, LDEVs, and pools with accelerated compression

After you have confirmed that the Saving % on used pool capacity is sufficient, you can create accelerated compression-enabled parity groups, LDEVs, and pools.

Use the following workflow to create accelerated compression-enabled parity groups, LDEVs, and pools

CautionWhen you create LDEVs in an accelerated compression-enabled FMC parity group, you must add all LDEVs you create to the same pool. If you do not add an LDEV to the pool, data recovery for a drive failure might fail. Make sure that you register all LDEVs to the pool.

During data recovery, LDEVs that were not added to the pool will be recovered to the FMC drive that was replaced when a failure occurred. In this case, the size of data to be recovered becomes greater than the original size, and the following might occur:

  • The pool reduction ratio might become very low.
  • The capacity of the FMC drive to which data is copied runs out, and data recovery might fail.
  • The FMC drive capacity runs out, the write pending rate of cache remains high, and access to the storage system might be lost.

After data recovery, if you continue operation without formatting the following LDEVs, the FMC drive capacity remains consumed. As a result, the FMC capacity runs out, the write pending ratio of cache remains high, and access to the storage system might be lost:

  • LDEVs used as normal volumes before the accelerated compression function is enabled
  • LDEVs deleted from the pool because of the reduction of the pool capacity

To use LDEVs that are not registered to a pool, you must format them before the deletion. Quick format cannot be performed.

  1. Check whether FMC parity groups are already used.

    If yes, go to step 2.

    If no, go to step 3.

  2. Enable the accelerated compression function for an existing parity group. Use the Edit Parity Groups window to enable accelerated compression.
  3. Use the new FMC capacity to create accelerated compression-enabled parity groups. Use the Create Parity Groups window to create parity groups.
  4. Create LDEVs to be used as pool-VOLs. Use the Create LDEVs window to create LDEVs.

    Best practice is to create 2.99-TB LDEVs because this is the maximum capacity of a pool-VOL. Use the following formula to calculate the recommended value of the total LDEV capacity to be defined for one parity group: 

    Total LDEV capacity = FMC-capacity-of-the-parity-group / (100% - (Saving % - 10%)) / 110%

    The buffers in the formula are as follows:

    • - 10%: Buffer representing the expected increase in capacity used because of degradation in the Saving %
    • / 110%: Buffer representing the expected increase in capacity used to store management information of the storage system

    Note
    • If the 14-TB FMC drive is used and the Saving % exceeds 75%, apply 75% for "Saving %" in the formula for total LDEV capacity.
    • The total capacity of LDEVs that can be created from a parity group of 14-TB FMC drives is equal to the total capacity of LDEVs that can be created from a parity group of 7-TB FMC drives (for example, up to 8x expansion for the 7-TB parity group but only 4x expansion for the 14-TB parity group). This is because the virtualized total capacity of LDEVs that can be created from a 14-TB FMC parity group exceeds the capacity of the Parity Group table.

    For example, when the 3.2-TB FMC is used in a 3D+1P configuration and the estimated Saving % is 40%, the number of required LDEVs is calculated as follows:

    • Calculate the total capacity of the LDEVs to be created as follows: 
      Total LDEV capacity = 9.6 TB / (100% - (40% - 10%)) / 110% = 12.5 TB
    • Calculate the number of LDEVs as follows. The value enclosed in ceiling( ) must be rounded up to the nearest whole number. 
      ceiling(12.5 TB / 2.99 TB ) = 5

      If the capacity of each LDEV is 2.99 TB, 5 LDEVs are required.

    NoteIf you use multiple parity groups, best practice is to configure the same basic usable capacity expansion rate for each parity group. Use the following formula to calculate the basic usable capacity expansion rate of the parity groups: 
    Basic usable capacity expansion rate of the parity groups = Total-capacity-of-the-LDEVs-created-from-the-parity-groups / FMC-capacity-of-the-parity-groups
  5. Create or expand the pool, and then add all of the created LDEVs to the same pool.
    Note Add all of the LDEVs that were created from a single parity group to the same pool.
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