Virtual Storage Device (VSA):
Virtual Storage Appliance (VSA) is a storage controller that works on a virtual machine to create shared storage space without the need for additional hardware. It offers either file level or block level storage for the network.
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VSA is not a physical device connected to any specific device. Uses the host system's local disk for storage as a virtual disk or can directly access local physical drives. In fact, VSA creates a virtual storage area similar to network storage by integrating directly connected capacity on each physical host. Small businesses that need redundancy and high availability of shared storage and also large organizations that transfer data between different arrays are good candidates for a virtual storage device.
Differences between NAS, SAN and DAS:
NAS differs from SAN in the way they distribute storage to other devices, which means that the main difference between these two is the access protocols. NAS supports data storage under the CIFS and NFS (file level) protocols, while the access protocols in SAN are ISCSI and Fiber Channel (block level).
On the other hand, DAS may appear to be faster than any other storage method. Since it needs to transfer the data over the network, all the data will be transferred on a dedicated connection between the server and the storage device. However, due to the high speed connection of the Fiber Channel, in some cases SAN offers a higher speed compared to the speed provided by DAS. Also, because SAN separates storage from servers and consolidates them into a location accessible through any application, it offers better storage flexibility, availability and performance compared to DAS for large enterprises. Therefore, due to the enhanced SAN storage performance, organizations will not need additional storage devices, which saves money, although SAN devices can be costly. Moreover, separating the storage device from the servers allows the computing resources on the servers to perform other tasks not related to storage.
On the other hand, the difference between DAS and Network Attached Storage (NAS) is that a DAS device communicates directly with a server without a network connection. DAS provides good performance, but there are limitations such as the number of servers it can access. DAS storage must be located near the server on the same shelf. The advantage of NAS over DAS is that it simplifies sharing files between multiple users. The main difference is how the computer or server handles a specific volume. If the server processor were responsible for managing the attached storage, it would be a kind of DAS and if the computer or server treats the attached storage like another computer, sharing its data over the network, it's a NAS.