Network Storage Systems Designs
After looking at the different types of storage communication (block-based and the file-based) and understanding the different protocols used in that communication, we need to look at the different designs of storage communication. Depending on the needs of the users, the applications, and the environment in which they exist, different designs can be used:
- Direct-attached storage (DAS)
- Network-attached storage (NAS)
- Storage area network (SAN)
Each of these designs has its advantages and disadvantages, which makes them suitable for different needs and applications.
Direct-Attached Storage (DAS)
Direct-attached storage (DAS) is a storage system that is directly attached to the host, available to it, and the only way of accessing it will be through the operating system/CPU of the host. The communication is based on using different protocols and buses, such as SCSI with Parallel SCSI, Serial Attached SCSI (SAS), NVMe, USB, ATA, SATA, and so on. It is based on host bus adapter, which communicates with the storage system, supporting the appropriate protocol. This means that when DAS is discussed, it does not mean only the local hard drives, but also optical drives, or external enclosures with one or multiple disks that are at the disposal only to that host. One of the characteristics of the DAS is that the relationship between the host and the storage is 1:1. The concept of the DAS is illustrated on Figure 10-5.
Figure 10-5 Direct-Attached Storage
On one hand, this is an advantage, as the storage provides all its resources to the host and the access is controlled and secured, which also can be described as captive storage. But, on the other hand, this can be looked at as a disadvantage, as the resources of the storage system are available to and controlled by a single host, which means storage system capacities can be underutilized.
The DAS can also be used by other hosts across a network if it is shared by its host. Still, it’s the host that is in control of who uses the resources of the storage system and how they are used. Implementing such a sharing approach can be complex and difficult to monitor and manage.
The resources of the storage system are limited, and even though in certain situations the host might not be able to utilize all of them, there might be situations in which the host will need more resources and the storage system cannot provide more than it has. This means that scalability with the DAS is limited. Using DAS for the purposes of creating and storing backups for multiple hosts is not a good application.