Like the free-for-all model of Figure 3, the per-drive-masking model of Figure 4 might be usable in certain controlled environments. As long as the number of disk drives connected is small (for example, the 1 to 15 drives SCSI supported), then this model is quite workable. Once the system grows beyond the bounds of directly connected disk drives, however, the complexity of this mechanism becomes cumbersome.
Furthermore, implementing the software to support a proprietary mechanism for a dozen or so disk drives is probably irritating but not prohibitive. Extending that software to tens, hundreds, or thousands of disk drives is likely more than any sane developer would take on.
Luckily, SAS provides a standard mechanism for access control, called zoning, which is nearly perfect for storage virtualization. SAS zoning is very analogous to similarly named mechanisms in Fibre Channel and other storage-area network (SAN) technologies.
SAS is a point-to-point serial protocol designed as the successor to parallel SCSI, which utilizes devices called expanders to enable the connection of additional devices. A typical SAS host adapter might implement eight ports, allowing the direct connection of eight disk drives. (Actually, SAS disk drives may use multiple ports to provide additional bandwidth, so those eight ports could even be fully utilized by having four higher-performance two-port disk drives attached.)
To provide more connectivity, SAS expanders would be used as shown in Figure 5, ignoring the colors for the moment. Each of these expanders is logically a switch, though without the high dollar cost associated with Fibre Channel switches. SAS expanders can optionally support a zoning capability, providing a means to limit access from specified hosts to specified targets, such as disk drives.
In SAS zoning, access is controlled per connection point on the expander (called a “PHY” in SAS-speak). Each expander maintains a table of which PHYs can communicate with other specific PHYs on that expander. By manipulating these tables on its zoning expanders, a SAS system can provide full access control.
SAS zoning is configured via special SAS messages that extend the existing SCSI Management Protocol inherent in SAS already. The protocol already comprehends the idea of a protected “supervisor” as the only agent allowed to reconfigure the zones.
Because SAS zoning is done per connection point, adding or removing devices automatically triggers zone re-analysis and potentially zone reconfiguration. Thus, new disk drives may be added to a zone without disrupting other zones—or even alerting them that the system configuration changed.
Several articles could be written about SAS zoning alone. But for the purposes of this article, suffice it to say that zoning provides full host to disk isolation and access control (Fig. 5, again), with colors representing each zone.
Following these steps, it’s clear that mapping SAS zones to the SIs of PCI Express I/O virtualization provides a full-featured implementation of storage virtualization. Figure 6 shows the full picture of a SAS IOV controller. The SAS controller provides one or more logical SAS expander(s) internally with slight tweaks to map SIs as if they were PHYs. Each SI then sees only a portion of the total storage pool, without the need for a software intermediary filter. Furthermore, this has been accomplished using existing standardized mechanisms.
While this example used a plain SAS controller, a SAS RAID controller could be used as well. Such a RAID controller would likely present its RAID sets as if they were simple disk drives behind the same type of internal logical SAS expander as was used in the controller.
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