What conv=fsync does in dd command-line utility

Question

I am copying yocto build linux image to sd-card from Ubuntu 14.04 system. The command is:

$ sudo dd bs=4M if=filename.rpi-sdimg of=/dev/sdX conv=fsync

What does mean here of

conv=fsync

I red from here

Synchronize output data and metadata just before finishing. This forces a physical write of output data and metadata.

But I did not understand by this line. What is mean by synchronization, metadata and what will be the effect if I do not include it in the command.

How is it different from fdatasync which only synchronizes output data.

Answer 1

Some devices use buffers and caches in order to improve their throughput and latency performance. This command makes the device flush its buffers and caches so that if the device is removed the data is written to it before the operation is marked as complete and control passed back to the terminal prompt.