Function to add entry in wp_rtamazon_s3_media table for all attachments.

s3converter.php

<?php
/**
 * set /?do=now at the end of site your URL to fire this script.
 */

/**
 * Function return attachment data needed for s3 table upload.
 *
 * @return array $attachment_data_array Attachment data array.
 */
function get_attachments_s3_array() {

	$args = array(
			'post_type' => 'attachment',
			'post_status' => 'inherit',
			'posts_per_page'  => -1,
			'post__in' => array(2230)
		);
	$final_attachment_data = array();
	$attachment_data = get_posts( $args );

	if(!empty($attachment_data )){
		$attachment_data_array = array();
		foreach ( $attachment_data as $key => $value ) {
			$single_attachment_data['blog_id'] = 1;
			$single_attachment_data['media_id'] = $value->ID;
			$url_host = parse_url( $value->guid );
			$single_attachment_data['obj_key'] = $url_host['path'];
			$single_attachment_data['wp_url'] = $value->guid;
			$bucket = rtamazon_s3_get_bucket_name();
			$rtawss3_client = new RTAWSS3_Client();
			$key = $url_host['path'];
			$s3_url = $rtawss3_client->rtawss3_get_object_url( $bucket, $key );
			$single_attachment_data['s3_url'] = $s3_url;
			$single_attachment_data['bucket'] = $bucket;
			$single_attachment_data['parent_id'] = null;
			$single_attachment_data['media_size'] = null;
			$single_attachment_data['status'] = 'moved';
			//set parent attachment data.
			$attachment_data_array[] = $single_attachment_data;

		}
		if(!empty($attachment_data_array)){
			//set child attachment data.
			$final_attachment_data = add_child_attchments( $attachment_data_array );
		}
	}

	return $final_attachment_data;
}

/**
 * Render child attachment data based on given attachment array.
 *
 * @param array $attachment_data parent attachment data to be store in s3 DB table.
 */
function add_child_attchments( $attachment_data ) {
	if ( ! empty( $attachment_data ) ) {
		foreach ( $attachment_data as $parent_key => $parent_value ) {
			$child_attachment_array = array();
			if ( ! empty( $parent_value['media_id'] ) ) {
				$attachment_meta_data = wp_get_attachment_metadata( $parent_value['media_id'] );
				if ( ! empty( $attachment_meta_data['sizes'] ) ) {
					foreach ( $attachment_meta_data['sizes'] as $_meta_key => $_meta_value ) {
						$child_attachment_array = $parent_value;
						$child_attachment_array['media_id'] = null;
						$child_attachment_array['media_size'] = $_meta_key;
						$child_url_array = explode( '/', $parent_value['wp_url'] );
						array_pop( $child_url_array );
						array_push( $child_url_array, $_meta_value['file'] );
						$child_url = implode( '/',$child_url_array );

						$child_attachment_array['wp_url'] = $child_url;
						$url_host = parse_url( $child_url );
						$child_attachment_array['obj_key'] = $url_host['path'];
						$bucket = rtamazon_s3_get_bucket_name();
						$rtawss3_client = new RTAWSS3_Client();
						$key = $url_host['path'];
						$s3_url = $rtawss3_client->rtawss3_get_object_url( $bucket, $key );
						$child_attachment_array['s3_url'] = $s3_url;
						$child_attachment_array['parent_id'] = $parent_value['media_id'];
						$attachment_data[] = $child_attachment_array;
					}
				}
			}
		}
	}
	return $attachment_data;

}

/**
 * Function insert data in amazon s3 table and set /?do=now in your URL to fire this script.
 */
function save_amazons3_attachment_data() {
	if(isset($_GET['do']) && $_GET['do'] == 'now'){
		global $wpdb;
		$table_name = $wpdb->prefix . 'rtamazon_s3_media';
		$attachment_data = get_attachments_s3_array();
		if(!empty($attachment_data)){
			foreach ( $attachment_data as $key => $value ) {
				$format = array( '%d','%d','%s','%s','%s','%s','%d','%s','%s' );
				$wpdb->insert( $table_name, $value, $format );
			}
		}
	}
}

add_action( 'init','save_amazons3_attachment_data' );