I have an EC2 instance with data I want to sync to a mounted, but remote, volume, as a backup.

rsync seems like the way to go with this, so as a test I took my test file (a Postgres pg_dump file) and used rsync -v to copy it to the mounted volume:

[ec2-user work]$ rsync -v dump.sql.1 ../backup/dump.sql
dump.sql.1

sent 821704315 bytes  received 31 bytes  3416650.09 bytes/sec
total size is 821603948  speedup is 1.00

Then, I ran it again, expecting to see minimal sent/received numbers because it would just be checksums. Instead...

[ec2-user work]$ rsync -v dump.sql.1 ../backup/dump.sql
dump.sql.1

sent 821704315 bytes  received 31 bytes  3402502.47 bytes/sec
total size is 821603948  speedup is 1.00

I'm new to rsync so perhaps I'm missing something, but isn't the idea that the source and destination files are checked for differences, and then a patch is generated and applied to the destination? Why is this not reducing the amount of data 'sent' to just the size of the checksums?

Some background if it's relevant:

the mounted volume is using s3fs, mounted with s3fs <bucketname> backup.