These are variants on S3. S3 uses the "pure" S3 interface, e.g. using
Multi-Part-Upload. The "plus" variant enables EMC-extensions in the aws4c
library. This allows the N:N case to use "append", in the case where
"transfer_size" != "block_size" for IOR. In pure S3, the N:N case will
fail, because the EMC-extensions won't be enabled, and appending (which
attempts to use the EMC byte-range tricks to do this) will throw an error.
In the S3_EMC alg, N:1 uses EMCs other byte-range tricks to write different
parts of an N:1 file, and also uses append to write the parts of an N:N
file. Preliminary tests show these EMC extensions look to improve BW by
~20%.
I put all three algs in aiori-S3.c, because it seemed some code was getting
reused. Not sure if that's still going to make sense after the TBD, below.
TBD: Recently realized that the "pure' S3 shouldn't be trying to use
appends for anything. In the N:N case, it should just use MPU, within each
file. Then, there's no need for S3_plus. We just have S3, which does MPU
for all writes where transfer_size != block_size, and uses (standard)
byte-range reads for reading. Then S3_EMC uses "appends for N:N writes,
and byte-range writes for N:1 writes. This separates the code for the two
algs a little more, but we might still want them in the same file.
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