7.6 iSCSI-to-SCSI Bridges

As with the Fibre Channel-to-SCSI bridge products reviewed in Chapter 5, iSCSI-to-SCSI bridges bring legacy SCSI tape and disk assets into an IP storage network. Given that 85 percent of organizations and enterprises still use direct-attached storage, the potential market for iSCSI-to-SCSI bridges is quite large. Over time, legacy SCSI disk arrays may be retired, primarily because of the relatively higher cost of maintenance and the scarcity of older SCSI disk drives. SCSI-based tape subsystems represent a substantial customer investment, however, and tend to stay on the books for a long time. iSCSI-to-SCSI bridges enhance the value of these assets, simply by virtue of sharing resources with new iSCSI servers and new SAN-enabled applications.

As shown in Figure 7-7, an iSCSI-to-SCSI bridge can be attached directly to the IP network. From the standpoint of the iSCSI servers, the tape target appears as another iSCSI resource, reported through SLP or iSNS discovery. From the standpoint of the SCSI tape device, the iSCSI servers appear as conventional SCSI hosts to which the tape device has parallel SCSI cabling attachment. Access to the tape resource is managed by a backup server installed somewhere within the IP infrastructure. This server launches backups for each server on a predetermined schedule.

Figure 7-7. Using an iSCSI-to-SCSI bridge to integrate SCSI resources in an IP SAN

The iSCSI-to-SCSI bridge may also contain an extended copy (third-party copy) agent to handle the reads and writes of data directly from iSCSI storage to the tape subsystem. This takes the iSCSI server out of the data path and reduces the total volume of traffic on the IP network. It also frees server CPU cycles for processing user requests.

iSCSI-to-SCSI bridges may also be targets of NAS appliances that support iSCSI in addition to NFS and CIFS file transport protocols. NAS appliances can thus serve files to workstations and servers while leveraging block I/O for more efficient backup of NAS storage over the IP network. In addition, some vendors are adding support for other disk interconnections such as Serial ATA (Advanced Technology Attachment), as well as simple virtualization capability so that disparate disks attached to the bridge can be managed as a single storage pool.