Update README.md

README.md

@@ -322,6 +322,14 @@ Each individual file size is limited to the volume size.
 
 All file meta information stored on an volume server is readable from memory without disk access. Each file takes just a 16-byte map entry of <64bit key, 32bit offset, 32bit size>. Of course, each map entry has its own space cost for the map. But usually the disk space runs out before the memory does.
 
+### Extensible to the cloud ###
+
+The local volume servers are much faster, while cloud storage has elastic capacity and more cost-efficient (usually free to upload, but relatively costly to access). SeaweedFS can take advantage of both local and cloud storage.
+
+Usually hot data are fresh and warm data are old. SeaweedFS puts the newly created volumes on local servers, and optionally upload the older volumes on the cloud. If the older data are accessed less often, this literally gives you unlimited capacity with limited local servers, and still fast for new data. 
+
+If the hot~warm data is split as 20~80, with 20 servers, you can achieve storage capacity of 100 servers. That's a cost saving of 80%! Or you can repurpose the 80 servers to store new data also, and get 5X storage throughput.
+
 [Back to TOC](#table-of-contents)
 
 ## Compared to Other File Systems ##