# Data Chunks Placement Optimization for Hybrid Storage Systems
## 2 Summary of Results
### Public Cloud Storage Cost Strategies  
Most of the vendors offer a wide array of different cloud storage services, and the price for each of those services varies from storage provider to storage provider. Because of the large variety of cloud storage services, different pricing models are employed by cloud vendors [10]. Nonetheless, most of the pricing models have a common substructure, namely: they charge for the used storage, the used outgoing traffic, and the amount of reading and write operations. Specifically, cloud vendors charge mostly for two types of data activity:  
* Data transfer cost [11]: Type of rate which is charging for every GB downloaded from the cloud or moved to another cloud facility. * Requests (API calls) cost [12]: Type of pricing for data operations within the storage, e.g., copying data or adding new objects. As a rule, all requests are paid, except commands for data deletion. Furthermore, most cloud providers use a block rate pricing model [13], where data storing prices decrease with storing more data. Because cloud providers have several geographical data centers (regions) [14], they use different cost calculation models for additional services (e.g., data migration), as well. Notably, those providers often offer reduced prices, as well, for data migration services between regions. Apart from the varying pricing, which is based on the regions, pricing models can also differ by storage technologies which are using by cloud vendors. Generally, there are three types of data storing and retrieving technologies which use by cloud providers. Those are \"_Hot Data_\", \"_Cold Data_\", and \"_Cool Data_\" [15; 16]. In comparison with traditional conventional storages, long-term storage solutions have cheap storage and high traffic prices. Moreover, this type of storage has a minimum storage duration, i.e., Billing Time Unit (BTU) [14]. BTU is independent of API calls charge, and it will not change, whether or not the data was migrated or deleted. Because long-term storages are used mostly for backup purposes, the _BTU_ cloud vendors has a charge for the minimum object size: _Billing Storage Unit (BSU). BSU_ is a standard pricing methodology for all types of data that have a smaller size than the _BSU_ size. _BSU_ size will be accepted as a minimum for all types of data, regardless the size of the data is less than that. While the pricing models used by cloud vendors are based on similar characteristics, there are also significant technological and conceptual differences between them. For instance, Amazon offers storage technologies, like Amazon Simple Storage Service (Amazon S3) and Amazon S3 Standard--Infrequent Access (IA) [17], with 30 days _BTU_ and a 128 KB _BSU_[18]. For these types of storages, Amazon applies block rate pricing model for the traffic, as well as for the storage. At the same time, Google also applies the block pricing model for their \"Google Cloud Storage\" but only for outgoing data transfer [19]. At the end of the paper, we provide more information about the pricing of these cloud storage systems. In our paper, we introduce a new framework for hybrid cloud systems. Our research is mostly focused on optimizing heterogeneous characteristics (throughput, storage cost, etc.) of hybrid cloud storage systems. The main goal of this investigation was to use the advanced performance of Network coding for resource allocation in a flexible manner between public and private cloud storage nodes, to increase upload/download speed, while minimizing the data upload/download and store cost. More explicitly, our approach is to store Network Coded objects in distributed cloud storage entities, e.g., Amazon S3 and Google GCS. In Network Coding, the objects are stored as different random linear combinations of the chunks in each cloud storage. This approach allows us to avoid an overwhelming bookkeeping process when transferring the data between nodes and additional delays from a highly loaded cloud storage.