Question

Lets say I have a hash of a hash e.g.

$data = {
    'harry' : {
         'age' : 25,
         'weight' : 75,
    },
    'sally' : {
        'age' : 25,
        'weight' : 75,
    }
}

1. What would the ‘usual’ way to store such a data structure (or would you not?)
2. Would you be able to directly get a value (e.g. get harry : age ?
3. Once stored could you directly change the value of a sub key (e.g. sally : weight = 100)

Answer 1

What would the ‘usual’ way to store such a data structure (or would
you not?)

For example harry and sally would be stored each in separate hashes where fields would represent their properties like age and weight. Then set structure would hold all the members (harry, sally, …) which you have stored in redis.

Would you be able to directly get a value (e.g. get harry : age ?)

Yes, see HGET or HMGET or HGETALL.

Once stored could you directly change the value of a sub key (e.g.
sally : weight = 100)

Yes, see HSET.