FlashArray performance and capacity management rely on "Write Groups"—logical groupings of drives that Purity uses to stripe data. A "Wide Write Group" typically consists of roughly 20 drives and offers better performance and capacity efficiency than smaller groups. However, Purity generally requires write groups to be contained within a single physical enclosure (chassis or shelf) to maintain failure domain boundaries and performance predictability.

In this scenario, the customer wants the new 91TB pack to form a wide write group. The chassis already has a 91TB group and a 45TB group. The shelf has one 45TB group. To create a wide write group with the new 91TB pack, it is best to pair it with an existing matching or compatible group in the same enclosure. Since the chassis is full (20 drives), you cannot simply add the new pack there without moving something.

The correct procedure is to move the 45TB write group from the chassis to the shelf . This frees up 10 slots in the chassis. The shelf now holds two 45TB groups (which is valid). The chassis now has the original 91TB group and 10 empty slots. The engineer can then install the new 91TB data pack into those empty chassis slots. Purity will then be able to merge or manage these two 91TB packs (the existing one and the new one) as a highly efficient configuration, potentially forming the requested wide write group or at least balancing the high-capacity drives within the controller chassis where bandwidth is highest. This physical rearrangement is necessary because write groups cannot span across the chassis-shelf interconnect.

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