NSFS | versioning | add comment and documentation about multipart upload version-id

Signed-off-by: nadav mizrahi <nadav.mizrahi16@gmail.com>

Author: nadavMiz

docs/design/NsfsVersioning.md

@@ -89,6 +89,12 @@ In order to support best effort on scale of these scenarios, for POSIX file syst
 5. else - unlink unique_tmp_path
 ```
 
+### Multipart upload version order
+According to AWS specifications, multipart upload version time should be calculated based on multipart upload creation time rather than completion time (see [AWS Documentation](https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpuoverview.html#distributedmpupload)).   
+On the other hand, for directory buckets, the object creation time is the completion date of the multipart upload  (see [AWS Documentation](https://docs.aws.amazon.com/AmazonS3/latest/userguide/s3-express-using-multipart-upload.html#s3-express-distributedmpupload)).
+There are performance issues for calculating the latest version after complete multipart whe using creation time.  
+In our design, due to the performance issues and to be aligned with AWS directory buckets, the version-id time is calculated based on completion time.
+
 ## OUT OF SCOPE
 ### TODO
 * Add GPFS design.

src/sdk/namespace_fs.js

@@ -1824,15 +1824,20 @@ class NamespaceFS {
         }
     }
 
-    // iterate over multiparts array - 
+    // iterate over multiparts array -
     // 1. if num of unique sizes is 1
     //    1.1. if this is the last part - link the size file and break the loop
     //    1.2. else, continue the loop
     // 2. if num of unique sizes is 2
     //    2.1. if should_copy_file_prefix
-    //         2.1.1. if the cur part is the last, link the previous part file to upload_path and copy the last part (tail) to upload_path  
+    //         2.1.1. if the cur part is the last, link the previous part file to upload_path and copy the last part (tail) to upload_path
     //         2.1.2. else - copy the prev part size file prefix to upload_path
     // 3. copy bytes of the current's part size file
+    // NOTE on versioning - according to general aws specifications, the version_id time should be based on when we created the upload.
+    // for directory buckets, on AWS, the object creation time is the completion date of the multipart upload
+    // on our design we decided to do it based on when the upload was completed.
+    // see https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpuoverview.html#distributedmpupload
+    // see https://docs.aws.amazon.com/AmazonS3/latest/userguide/s3-express-using-multipart-upload.html#s3-express-distributedmpupload
     async complete_object_upload(params, object_sdk) {
         const part_size_to_fd_map = new Map(); // { size: fd }
         let read_file;