Created Large File Handling (markdown)

Large-File-Handling.md

@@ -0,0 +1,40 @@
+In order to achieve high concurrency, SeaweedFS tries to read and write the whole file into memory. But this would not work for large files. 
+
+To support large files, SeaweedFS supports these two kinds of files:
+* Chunk data File
+* Chunk Manifest
+
+## Create new large file
+
+SeaweedFS delegates the effort to the client side. The steps are:
+
+1. split large files into chunks
+1. upload each file chunks as usual, with mime type "application/octet-stream". Save the related info into ChunkInfo struct.
+1. upload the manifest file with mime type "application/json", and add url parameter "cm=true".
+
+
+```
+type ChunkInfo struct {
+	Fid    string `json:"fid"`
+	Offset int64  `json:"offset"`
+	Size   int64  `json:"size"`
+}
+
+type ChunkList []*ChunkInfo
+
+type ChunkManifest struct {
+	Name   string    `json:"name,omitempty"`
+	Mime   string    `json:"mime,omitempty"`
+	Size   int64     `json:"size,omitempty"`
+	Chunks ChunkList `json:"chunks,omitempty"`
+}
+```
+
+## Update large file
+
+Usually we just append large files. Updating a specific chunk of file is almost the same.
+
+The steps to append a large file:
+
+1. upload the new file chunks as usual, with mime type "application/octet-stream". Save the related info into ChunkInfo struct.
+1. update the updated manifest file with mime type "application/json", and add url parameter "cm=true".