Sync command guide

AzCopy offers a convenient sync command to save on costs when transferring large amount of data. It enumerates both the source and destination directories, and compare the last modified times of the source and destination files, to see if they need to be transferred.

For a given file at the source, AzCopy would transfer it to the destination if:

1. it does not exist at the destination
2. the same file also exists at the destination, but its last modified time is earlier than the source file

It should be noted that for the sync command to work properly, the machine from which it runs should have a reasonably accurate system clock, since last modified times are critical in determining whether a file should be transferred. If there is a significant clock skew, the user is advised to not modify the destination too closely to running a sync command.