How to figure out that the data has been loaded is some rows when using Ext.data.operation and ajaxProxy?

the code is here:

Ext.define('Book', {
extend: 'Ext.data.Model',
fields: [
{name: 'id', type: 'int'},
{name: 'title', type: 'string'},
{name: 'pages', type: 'int'},
{name: 'numChapters', type: 'int'}
]
});

var ajaxProxy = Ext.create('Ext.data.proxy.Ajax', {
url: 'books.json',
model: 'Book'
});

var operation = Ext.create('Ext.data.Operation', {
action: 'read',
start: 0,
limit: 5
});

operation.read();

books.json contains 6 rows, when i try the code bellow, i get 6 rows, when i expected its load 5 rows.

ajaxProxy.read(operation, function () {
console.log(operation.response.responseText);
});