Hi,

I keep getting the message on one of my alpha nodes that the block cache might be too small. This started recently after I dropped and rebuilt an alpha node. I am not noticing any explicit performance issues but want to know if I should be concerned and if there is anything to fix this?

```
prod-dgraph-alpha-2 prod-dgraph-alpha W1102 10:48:49.036461 20 log.go:36] Block cache might be too small. Metrics: hit: 3942698 miss: 68352954 keys-added: 408518 keys-updated: 359210 keys-evicted: 396959 cost-added: 47445812686 cost-evicted: 47325553751 sets-dropped: 8935 sets-rejected: 7699 gets-dropped: 3686464 gets-kept: 67519808 gets-total: 72295652 hit-ratio: 0.05
prod-dgraph-alpha-2 prod-dgraph-alpha W1102 10:48:49.036546 20 log.go:36] Cache life expectancy (in seconds):
prod-dgraph-alpha-2 prod-dgraph-alpha -- Histogram:
prod-dgraph-alpha-2 prod-dgraph-alpha Min value: 0
prod-dgraph-alpha-2 prod-dgraph-alpha Max value: 19
prod-dgraph-alpha-2 prod-dgraph-alpha Count: 849
prod-dgraph-alpha-2 prod-dgraph-alpha 50p: 4.00
prod-dgraph-alpha-2 prod-dgraph-alpha 75p: 8.00
prod-dgraph-alpha-2 prod-dgraph-alpha 90p: 8.00
prod-dgraph-alpha-2 prod-dgraph-alpha [0, 2) 271 31.92% 31.92%
prod-dgraph-alpha-2 prod-dgraph-alpha [2, 4) 348 40.99% 72.91%
prod-dgraph-alpha-2 prod-dgraph-alpha [4, 8) 204 24.03% 96.94%
prod-dgraph-alpha-2 prod-dgraph-alpha [8, 16) 13 1.53% 98.47%
prod-dgraph-alpha-2 prod-dgraph-alpha [16, 32) 13 1.53% 100.00%
prod-dgraph-alpha-2 prod-dgraph-alpha --
prod-dgraph-alpha-2 prod-dgraph-alpha
```