2.4 Direct-Attached Storage

DAS expands server storage capacity. It provides easily configurable storage that looks local, or internal, to a server, yet has scalable capacity benefits of being outside the server itself. External storage can extend server life by providing not only additional capacity but a range of storage features, such as redundancy, that help protect data.

With DAS, servers access blocks of data. The file system and volume management layers are host-based. Other data storage functions implemented independent of the server, such as RAID, provide greater performance and leave the server free to focus on its priority tasks , such as application processing. Most external storage devices attach to servers via high speed parallel SCSI or Fibre Channel cabling. Traditionally, the speed difference between SCSI and Fibre Channel compared to LANs has been significant, although today's high-speed networking interconnects, such as Gigabit Ethernet, level the playing field. See Section 2.6.3, "SAN GUIDANCE: Comparing Ethernet/IP and Fibre Channel Fabrics," for a more complete overview of storage and networking interconnects.

However, even DAS has its limits. Adding additional storage usually requires incremental servers, and one server does not have direct access to another server's storage. This captive storage model makes resource sharing difficult and leaves servers susceptible to single points of failure. The basic DAS model is shown in Figure 2-6.

Figure 2-6. Direct-attached storage.

Reaching the limits of the captive storage model, solutions with additional scalability arose that networked storage together. This development took shape in the mid-1980s, well before the introduction of high-speed serial storage technologies such as Fibre Channel and iSCSI. Therefore, without a means to serialize the high-speed parallel SCSI interconnects between servers and DAS devices, the only other available network was the Ethernet and IP-based LAN. This led to the introduction of NAS.