Direct Attached Storage, DAS

History

Development of network technologies led to emergence of two network solutions for DWH – networks of storage Storage Area Network (SAN) for data exchange at the level of the blocks supported by client file systems and servers for data storage at the file Network Attached Storage (NAS) level. To distinguish traditional DWH from network one more retronym – Direct Attached Storage (DAS) was offered.

DAS, SAN and NAS appearing in the market consistently reflect the evolving chains of communications between the applications using data, and bytes in the carrier, containing these data. Once programs-applications were read and wrote blocks, then drivers as a part of the operating system appeared. In modern DAS, SAN and NAS the chain consists of three links: the first link – creation of RAIDs, the second – processing of the metadata allowing to interpret binary data in the form of files and records, and the third – services for providing data to the application. They differ on where and as these links are implemented. In a case with DAS DWH is "naked", it only gives an opportunity of storage and data access, and all the rest becomes on server side, starting with interfaces and the driver. With the advent of SAN providing RAID is transferred to the party of DWH, all the rest remains the same as in a case with DAS. And NAS differs in the fact that also metadata for ensuring file access are transferred to DWH besides, here the client needs only to support services of data.

Emergence of SAN became possible after in 1988 the Fibre Channel protocol (FC) was developed and in 1994 ANSI as the standard is approved. The term Storage Area Network is dated 1999. Over time FC gave way to Ethernet, and IP-SAN networks with connection on iSCSI gained distribution.

The idea of the network server of storage of NAS belongs to Brian Rendell from the Nyyukestla University and is implemented in machines on the UNIX-SERVER in 1983. This idea turned out so successful that it was picked up by a set of the companies, including Novell, IBM, and Sun, but finally replaced leaders of Netapp and EMC.

In 1995 Garth Gibson developed the principles of NAS and created object DWH (Object Storage, OBS). It began with the fact that separated all disk transactions into two groups, one entered executed more often, such as read and write, another more rare, such as transactions with names. Then he offered one more container in addition to blocks and files, he called it an object.

OBS differs in new type of the interface, it is called object. Customer services of data interact with metadata on object API (Object API). Not only data are stored in OBS, but also RAID is supported, the metadata relating to objects are stored and the object interface is supported. DAS, both SAN, and NAS, and OBS coexist in time, but each of access types corresponds to a certain data type and applications to a large extent.

In more detail about evolution of DWH read here.

Architecture of DAS

Scheme of connection of DAS

It is possible to carry their low cost to main advantages of DAS systems (in comparison with other solutions of DWH), simplicity of deployment and administration and also a high data exchange rate between storage system and the server. Actually, exactly thanks to it they won great popularity in a segment of small offices, hosting providers and small corporate networks. At the same time DAS systems have also the shortcomings to which it is possible to refer not optimal utilization of resources as each DAS system requires connection of a dedicated server and allows to connect at most 2 servers to the disk shelf in a certain configuration.

Advantages and shortcomings

Pluses

• Rather low cost. In fact this DWH represents the disk basket with hard drives which is taken out out of server limits.
• Simplicity of deployment and administration.
• High exchange rate between the disk array and the server.

Minuses

• Low reliability – at emergence of problems in network or failure of the server data become unavailable to all at once.

• The high latency caused by processing of all requests one server and the used transport (most often – IP).

• The high loading of network which is often defining scalability limits by adding of clients.

• The bad controllability – all capacity is available to one server that reduces flexibility of distribution of data.

• Low utilization of resources – it is difficult to predict required amounts of data, some DAS devices in the organization can have a surplus of the capacity (disks), at others it can not be enough – redistribution is often impossible or labor-consuming.

One of the main shortcomings of DAS technology of today's IT infrastructure — availability of the disk system associated with this server only via this device that leads to creation of certain "islands" of data. Besides, files cannot be shared by several servers, and for adding of disk storage modules to network servers should be transferred to a standalone mode. To resolve these problems and to provide redundancy and high readiness of data, the companies had to copy data on several servers and to use different RAID configurations.

Now, without regard to significant changes in volumes of storage and the nature of information systems, the solutions DAS remain at the heart of the invariable. As eventually requirements to storage systems grew, the companies just added disks to the servers and servers to the networks. What once was the practical solution led now to the fact that at the companies difficult infrastructure of storage systems and huge server farms which are harder and harder for managing was formed.

Sources

• Andrey Zakharov, Main systems of data storage and their feature
• Storage area networks

See Also

• Direct Attached Storage, DAS
• NAS
• Storage Area Network
• Data backup