Calculating the number of overwrite passes for FMDs

Because of the way space is allocated in FMDs, the dummy data that is used to overwrite the volume must exceed the capacity of the target volume. Therefore, before you shred a volume on FMDs, you need to calculate the number of times the system must overwrite the volume with dummy data.

Before you begin

You must have the Storage Administrator (Provisioning) role to perform this task.

Procedure

  1. Select an LDEV on the LDEVs tab in the Logical Devices window, and write down its capacity and the amount of data in the RAID configuration.

    Examples amounts of data in a RAID configuration:

    n of RAID5(nxD + 1P)

    n of RAID6(nxD + 2P)

  2. Use the following formula to calculate the number of times to overwrite the data (N):

    N = (User-capacity-for-Data x 2) / Capacity-of-target-volume When the capacity of the FMD is 1,600 GB, 1,600GB x n

    Example: The capacity of the FMD is 1,600 GB, the RAID configuration is 3D+1P, and the LDEV capacity is 880 GB.

    [(1,600 x 3) x 2] / 880 = 10.9 = 11 times

    Round up the value (10.9) to the nearest whole number. You need to overwrite the data 11 times.

    You can write dummy data up to eight times in one shredding operation. To write dummy data nine or more times, perform the shredding operation for the volume as many times as required.

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