Estimating FMC capacity for a net new pool

When purchasing new FMC capacity to create a pool, estimate the capacity to be purchased according to the following workflow:

The steps shown in the figure are described below.

 To estimate the FMC capacity to be used for an accelerated compression-enabled parity group, estimate a margin capacity in addition to the main capacity for storing data. Add approximately 20% of the required FMC capacity as the margin capacity. Margin capacity refers to the total expected increase in FMC capacity, which includes the following:

• Expected capacity increase for storing storage system management information

• Expected capacity increase for offsetting the degradation of the saving rate compared with estimated values

1. Estimate the required pool capacity.

Estimate the pool capacity required for user data in the same way you estimate capacity when creating a pool. If you are extending a pool, also estimate the additional capacity.

2. Estimate the data-size saving rate. Use either of the following methods:

• If data will be migrated into the FMC, use the hafdc2_estimator tool to estimate the data-size saving rate. The tool output will provide an estimated data-size savings rate that can be used to determine the capacity size needed. However, if the saving rate is less than 20%, the margin capacity offsets the effectiveness of saving the used capacity because you must estimate the capacity including the margin capacity in "Required FMC capacity to be purchased" in Step 3. In this case, we recommend using a parity group with accelerated compression disabled, and estimating the pool capacity by using the conventional method. Note that to use the hafdc2_estimator tool, you must install the tool on the server host. Please contact the HDS account team to get the hafdc2_estimator tool and instructions on how to use the tool.

• If new data is stored in the FMC, or if the hafdc2_estimator tool cannot be executed in the environment, we recommend that you consider setting the data-size saving rate to 0% and use conventional method to size capacity.

3. Estimate the required FMC capacity to be purchased.

When there is data to be migrated to the FMC, use the following formula to calculate the required FMC capacity to be purchased. Then purchase the FMC capacity that offers the estimated capacity and then enable accelerated compression to create parity groups, LDEVs, and pools:

Required FMC capacity to be purchased = Required pool capacity × (100 [%] - (Saving rate [%] - 10 [%])) × 110 [%]

The values of the margins in the above formula are as follows:

• - 10 [%]: Margin representing expected increase in capacity because of degradation in saving

• × 110[%]: Margin representing expected increase in capacity because of additional space required to store storage system management information

 When using Dynamic Tiering or Active Flash, if the Tier 1 is configured of FMC drives, use 1.2 times the calaculated required pool capacity, using the above formula. Use the following formula to calculate the value:

Required pool capacity = Required pool capacity estimated in Step 1 × 120 %

To prevent the depletion of the capacity reserved for writing due to the tier relocation, Dynamic Tiering or Active Flash assigns pages to secure 20% of the buffer area. However, if accelerated compression is disabled, Dynamic Tiering or Active Flash assigns pages without securing 20% of the buffer area. Therefore, if the same FMC (Tier1) capacity is applied, the stored data volume of which the accelerated compression is enabled is smaller than the stored data volume when accelerated compression is disabled.

To prevent this from happening, use 1.2 times the pool capacity estimated by procedure 1. The data volume at the same level in the case of the accelerated compression-disabled setting is located in FMC (tier 1).

If there is no data to be migrated, or if the hafdc2_estimator tool cannot be used, purchase FMC drives that offer the same capacity as necessary, considering the savings rate at 0%. Then, disable accelerated compression to create parity groups, LDEVs, and pools.