The short version would be:
I think these are really common sense options to use and probaby should be the default.
P = display the progress
D = hybrid of --specials and --devices so all special and device files will be copied as well.
r = recursive (otherwise rsync won't copy files deeper than the specified target)
p = preserve the permissions
h = human readable (usually MB or GB for the filesizes and MB/s or K/s for the transfer speeds
o = preserve the owner's ownership
g = preserve the group ownership
t = preserve times (you should especially for backups)
l = copy symlinks as symlinks (why wouldn't you?) *Note that if you are not copying an entire partition/drive you may have issues (where the symlink is copied and not the file you actually want, this option may not be a good idea for databackup but is mainly useful when preserving filesystems/migrating an OS etc...)
-a, --archive archive mode; equals -rlptgoD (no -H,-A,-
*As shown above you could replace r,l,p,t,og,D (basically everything by just using the -a) switch.
--exclude=/dev --exclude=/proc --exclude=/sys
Those are basic directories that usually result in rsync hanging when trying to copy an entire live filesystem.
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