copy-on-write used in Hadoop file system upgrade

[Lin Ma <li...@gmail.com>]

Hi guys,

I am reading from there for Hadoop handles file system upgrade (
http://www.storageconference.org/2010/Papers/MSST/Shvachko.pdf), it is
mentioned, "and block modifications during appends use the copy on write
technique",

   - I think it means old and new file system will keep only one copy using
   hard link to save space as long as the file has no change, and if during
   upgrade, the file changed (during append operation), then a new updated
   file will be created in order to save space. I am not sure if my
   understanding is correct?
   - Another question is, copy-on-write only used to save space for keeping
   snapshot information during file system upgrade? Or it has more broad use
   cases in HDFS?


regards,
Lin