Direct-attached storage devices can be attached to a host server via SCSI, Fibre Channel (FC), iSCSI or other types...
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of connection, but sharing is typically limited to a small number of host connections or ports. Technically, tape and optical devices can be directly attached to a system and can be considered DAS as well, but we will focus on disk storage for this article.
Ten years ago, direct-attached storage devices were the most common type of disk storage for open systems. Servers came with a certain number of internal hard drives and when additional data storage was required, a RAID controller card was added to the system to attach an external disk enclosure using a cable (e.g., SCSI or SSA). But as the amount of data stored by systems grew, direct-attached storage revealed some notable limitations such as scalability and the lack of redundant components. However, the biggest limitations with DAS were its inability to distribute unused storage between servers with extra capacity and its servers needing more capacity. With DAS, enterprise data storage users often ended up having to buy extra storage for a system in need of capacity while many other systems had gigabytes of unused disk space.
Centralized and network shared storage like network-attached storage (NAS) or storage area networks (SANs) then stepped onto the playing field. With centralized shared disk storage, capacity could now be pooled and made seamlessly accessible to multiple hosts via an IP network or by the way of a Fibre Channel network. With these conveniences, many people asked: Why would anyone still want to use DAS? For enterprise customers with hundreds of servers, the ability to pool unused disk storage can generate economies of scale and offer a lot of new data protection options such as array-based data replication, snapshots, remote mirroring, etc. However, SMB storage administrators thought differently about these options. For example, it is hard to convince a storage manager responsible for 20 servers to purchase a NAS or SAN solution when the two biggest storage consumers are the email and file servers.
In fact, DAS is still a viable option for many SMBs. There are many instances where there are simply not enough systems requiring external storage as mentioned above to justify a SAN or NAS solution. Companies like Dell with its MD3000 or Aberdeen's DAS-1002sa offer data storage arrays that can support up to four direct-attached hosts starting at $6,000 to $7,000. There are also organizations with remote offices that use local file servers with DAS to avoid network performance issues associated with a central data repository.
Another argument for centralized storage focuses around the fact that IT environments wanting to take full advantage of server virtualization through dynamic workload relocation between physical hosts must implement shared storage. But once again, many SMBs might not be swayed with this argument for shared storage; it is difficult to convince smaller IT shops to spend the money they just saved virtualizing 10 servers on a shared network storage solution. Although there are some benefits to including server virtualization as part of a disaster recovery (DR) strategy, many smaller organizations still feel they can recover their data without the automation provided by products such as VMware's VMotion, which requires shared storage. Instead, SMBs opt for a manual recovery process. A reduced number of critical systems allows them to consider and choose a direct-attached storage device instead. Even in the context of data backup, the argument that array-based replication keeps processing and I/O traffic off the host has less impact on smaller environments due to fewer critical systems needing that level of data protection.
Security and direct-attached storage devices
Any discussion on data storage would not be complete without touching on security. The ability to make data storage available through a network or any kind, whether via IP, iSCSI or Fibre Chanel Protocol (FCP) created a significant exposure in comparison to the relative security to SCSI of serially attached storage. While this theory has some merit, a well-secured SAN can be as difficult (or easy) to penetrate as a well secured IP network or even the server to which the DAS is connected. This effectively puts DAS and shared storage on an even playing field in term of security; both require proper protection and vigilance.
It is obvious that direct-attached storage devices no longer meet the requirement of enterprise-class IT storage due to constantly growing data storage requirements, the need for rapid provisioning and the development of advanced server and storage virtualization technology options. However, the market for DAS still exists. According to recent U.S. Department of Labor statistics, small firms with fewer than 500 employees represent over 99% of the 29.6 million businesses in the United States. While a good portion of these firms might be independent contractors and very small companies making little to no use of IT, it still leaves a significant number of cost-conscious SMBs that might opt for direct-attached storage rather than the pricier NAS or SAN solutions. This is another good indication that DAS in not going to disappear anytime soon.
About this author: Pierre Dorion is the data center practice director and a senior consultant with Long View Systems Inc. in Phoenix, Ariz., specializing in the areas of business continuity and DR planning services and corporate data protection.