+ // In AWS regions that use V4 signatures, we need to
+ // provide ContentSHA256 up front. Otherwise, the S3
+ // library reads the request body (from our buffer)
+ // into another new buffer in order to compute the
+ // SHA256 before sending the request -- which would
+ // mean consuming 128 MiB of memory for the duration
+ // of a 64 MiB write.
+ opts.ContentSHA256 = fmt.Sprintf("%x", sha256.Sum256(block))