[[!comment format=mdwn
 username="joey"
 subject="""comment 3"""
 date="2015-07-02T16:43:03Z"
 content="""
Not knowing how S3's backend is implemented, I have to assume that, in
order to get the advertised number of 9's of reliability, it involves some
replication of data, as well as some method to detect if a bit has flipped
or a drive has died, and recover.

This is requesting that git-annex ask S3 for a md5sum, and compare it
against a md5sum that it, presumably, keeps track of locally. If the two
are different, git-annex could tell that S3 has lost data. But, git-annex is
in a much worse position to tell if S3 has lost data then S3 itself is.
It seems very unlikely that this extra checking would ever detect a problem
that S3 didn't itself detect and fix (or in the 0.00001% case, 
fail to fix and delete the lost file?)

Bit flips during transfer seem more likely than that. `poContentMD5` could
help guard against those.
"""]]