Update 1.resource-preparations.md (#1927)

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

@@ -241,7 +241,7 @@ 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` * 7.5 * 120%      |For more information, see [Edge partitioning and storage amplification](../1.introduction/3.nebula-graph-architecture/4.storage-service.md).|
-| 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).|
+| 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 20 (both including). Its value depends on the disk performance. Use 20 for SSD and 2 for HDD.|
 
 
@@ -263,6 +263,9 @@ Users can estimate the memory, disk space, and partition number needed for a Neb
 
         Users can decrease the memory size occupied by the bloom filter by adding `--enable_partitioned_index_filter=true` in `etc/nebula-storaged.conf`. But it may decrease the read performance in some random-seek cases.
 
+!!! caution
+
+    Each RocksDB instance takes up about 70M of disk space even when no data has been written yet. One partition corresponds to one RocksDB instance, and when the partition setting is very large, for example, 100, the graph space takes up a lot of disk space after it is created.
 
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 ## Optimize the configuration for HDD