MySQL Database Design – Improving for Monitoring Applications

I need assistance with my DB design, I have a monitoring script that produce the following data every minute , and produce stats about virtual machine performance.

Array
(
    [timestamp] => 2015-05-05T18:11:00Z
    [vm_name] => i-2-20-VM
    [memory_internal_free] => 3516900
    [vbd_xvdd_read] => 0
    [vbd_xvdd_write] => 0
    [vif_0_rx] => 0
    [vif_0_tx] => 0
    [memory] => 4294967296
    [vbd_xvda_read] => 0
    [vbd_xvda_write] => 0
    [cpu0] => 0.000100
    [cpu1] => 0.000100
)
Array
(
    [timestamp] => 2015-05-05T18:11:00Z
    [vm_name] => i-2-24-VM
    [memory_target] => 4294967296
    [vbd_xvdb_read] => 0
    [vbd_xvdb_write] => 0
    [vbd_xvda_read] => 0
    [vbd_xvda_write] => 0
    [vif_0_rx] => 0
    [vif_0_tx] => 0
    [memory] => 4294967296
    [cpu0] => 0.000100
    [cpu1] => 0.000100
)

I'm trying to store the following info into my database, I have create single table

I have created table as follows

CREATE TABLE `md_metrics_status` (
  `id` int(100) NOT NULL AUTO_INCREMENT,
  `timestamp` datetime NOT NULL,
  `vm_name` varchar(50) DEFAULT NULL,
  `memory_target` varchar(255) DEFAULT NULL,
  `vif_2_rx` varchar(255) DEFAULT NULL,
  `vif_2_tx` varchar(255) DEFAULT NULL,
  `memory` varchar(255) DEFAULT NULL,
  `vbd_xvda_read` varchar(255) DEFAULT NULL,
  `vbd_xvda_write` varchar(255) DEFAULT NULL,
  `vbd_xvdd_read` varchar(255) DEFAULT NULL,
  `vbd_xvdd_write` varchar(255) DEFAULT NULL,
  `vif_1_rx` varchar(255) DEFAULT NULL,
  `vif_1_tx` varchar(255) DEFAULT NULL,
  `vif_0_rx` varchar(255) DEFAULT NULL,
  `vif_0_tx` varchar(255) DEFAULT NULL,
  `cpu0` varchar(255) DEFAULT NULL,
  `cpu1` varchar(255) DEFAULT NULL,
  `vif_3_rx` varchar(255) DEFAULT NULL,
  `vif_3_tx` varchar(255) DEFAULT NULL,
  `memory_internal_free` varchar(255) DEFAULT NULL,
  `vbd_xvdb_read` varchar(255) DEFAULT NULL,
  `vbd_xvdb_write` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=162339 DEFAULT CHARSET=utf8

Now this table has grown so large, and searching for records between specific timestamps is very slow.

I'm unable to add an index on timestamp field, because I get many duplicate records, as you can see in the example.

What is wrong with this table design? how can I improve this setup?

I have full control over the database, and over the script that produces the data.

Your input is highly appreciated.

Thank you