describe the mapping

Directories-and-Files.md

@@ -51,8 +51,8 @@ SeaweedFS wants to make as small number of disk access as possible, yet still be
 
 We can take the following steps to map a full file path to the actual data block:
 
-1. (file_parent_directory, fileName) => meta data of attributes and a list of file_id. Implemented by "weed filer" server.
-2. file_id => data_block. Implemented by default SeaweedFS master and volume servers.
+1. map <file_parent_directory, fileName> to <attributes, a list of file_id> Implemented by "weed filer" server.
+2. map <file_id> to <data_blocks>. Implemented by default SeaweedFS master and volume servers.
 
 #### Architecture
 
@@ -62,7 +62,7 @@ For file reads:
 3. Filer read from volume servers and pass along to the read request.
 ![](FilerRead.png)
 
-For writes:
+For file writes:
 1. Client stream files to Filer
 2. Filer uploads data to Weed Volume Servers, and break the large files into chunks.
 3. Filer writes the metadata and chunk information into Filer store.