S3piegel is a tool to mirror two AWS S3 folders. It's a bit like running

aws s3 sync s3://source s3://dest

except that it'll preserve object metadata. As the aws tool, S3piegel operates concurrently: while the listings of both the source and destination folders are sequential, the actual copying of files is done in parallel.

S3piegel uses the copy function of S3, the data is therefore only transiting inside AWS and not through your connection. It'll skip files that already exist in the destination folder (comparing only the name for now), unless you tell it not to.

S3piegel is a standard Go tool, and is "go-gettable":

go get github.com/abustany/s3piegel

Simply run

s3piegel s3://source s3://dest

run with -help to see the list of all options.