Sorry to come late to the review, but I would have liked to see a mention of how HDFS rename updates the modification time of both the source and the destination folder (though not the modification time of the renamed file itself).
Also, regarding this:
Object stores have a significantly simpler view of time:
+ A file's modification time is always the same as its creation time.
This makes it sound like this section covers all object stores, but the statement about modification time is not necessarily true universally. For example, on WASB, the FileStatus on read is always populated with the last modified time field of the blob as reported by the Azure Storage service. I think any kind of modification of the blob will result in a change in that value. I specifically tested hadoop fs -chmod against WASB, and it updated the blob's modification time, which is different from HDFS. Out-of-band blob modifications directly through the Azure Storage service, bypassing the FileSystem API, could be another source of perceived changes in the last modification time.
I expect this is not consistent across services, and therefore it's unlikely we can make accurate statements in the file system spec beyond just saying "it's different."
Please feel free to address this either by reverting and revising or filing a new JIRA to track an addendum.