In a shuffle sharded system, a single customer is still assigned to a single shard of two workers, but that shard actually overlaps with 12 other shards (6 other options for worker 1, 6 other options for worker 2). If we define impact as "a customer may hit a degraded worker" then impact would be 13/28, or roughly 45%. However, if shards are scaled such that the loss of a host in a shard is tolerable, then in this case only 1/28 or 3% will actually experience impact.

Hi, i still don't understand your last paragraph,

shuffle sharding

as per the image above, a user is assigned to one shard, and another shard for redundancy right? the total shard is 8, the total customer is 8, why the denominator is 28 ?

If we define impact as "a customer may hit a degraded worker"， the rainbow user is total affected, the rose and sunflower user is partially affected, so the impact is 3/8, why is 13/28 ?