Update upgrade docs for 3.0.0 (#1069)

docs-2.0/4.deployment-and-installation/1.resource-preparations.md

@@ -210,7 +210,7 @@ Users can estimate the memory, disk space, and partition number needed for a Neb
 | Resource      |Unit| How to estimate      |Description|
 |:--- |:---|:--- |:---|
 | Disk space for a cluster               |Bytes| `the_sum_of_edge_number_and_vertex_number` * `average_bytes_of_properties` * 6 * 120%      |-|
-| Memory for a cluster     |Bytes| [`the_sum_of_edge_number_and_vertex_number` * 15 + `the_number_of_RocksDB_instances` * (`write_buffer_size` * `max_write_buffer_number` + `rocksdb_block_cache`)] * 120% |`write_buffer_size` and `max_write_buffer_number` are RocksDB parameters. For more information, see [MemTable](https://github.com/facebook/rocksdb/wiki/MemTable). For details about `rocksdb_block_cache`, see [Memory usage in RocksDB](https://github.com/facebook/rocksdb/wiki/Memory-usage-in-RocksDB#block-cache).|
+| Memory for a cluster     |Bytes| [`the_sum_of_edge_number_and_vertex_number` * 16 + `the_number_of_RocksDB_instances` * (`write_buffer_size` * `max_write_buffer_number` + `rocksdb_block_cache`)] * 120% |`write_buffer_size` and `max_write_buffer_number` are RocksDB parameters. For more information, see [MemTable](https://github.com/facebook/rocksdb/wiki/MemTable). For details about `rocksdb_block_cache`, see [Memory usage in RocksDB](https://github.com/facebook/rocksdb/wiki/Memory-usage-in-RocksDB#block-cache).|
 | Number of partitions for a graph space |-| `the_number_of_disks_in_the_cluster` * `disk_partition_num_multiplier`                                                                                                               |`disk_partition_num_multiplier` is an integer between 2 and 10 (both including). Its value depends on the disk performance. Use 2 for HDD.|
 
 * Question 1: Why do we multiply the disk space and memory by 120%?