Disk map decisions
There are two ways of implementing virtual storage: symmetrical and asymmetrical. In symmetrical storage virtualisation, version B in the diagram below, the control device sits astride the data path and all data requests must pass through it. Asymmetrical virtualisation (version A) places the control device alongside the data path. The virtualisation theory is the same; the difference is where the virtual disk map is stored.
Symmetrical virtualisation is typified by devices running Datacore's SANsymphony software, such as the Gadzoox Axxess SMA. This is an Intel-based rack-mounted server with dual or quad Pentium III Xeon processors, running Windows NT, but only the kernel and storage services are loaded.
While the Axxess SMA connects directly into a Gadzoox Capellix switch using two or more host bus adapter cards (HBAs), the two are tightly integrated. The SMA uses zoning to separate the servers and storage, with only the Axxess device seeing both. This has backup advantages, as the SMA knows what is stored where and can replicate it if necessary. It can also aggregate the storage allocated to each operating system environ-ment and back it up as a block.
The symmetrical approach can affect network performance. Every I/O must go through the SMA, because it sits on the data path between the servers and the storage pool. Axxess therefore incorporates up to 2GB of cache to reduce latency. But a fabric can only have one active Axxess SMA, though it can be connected to multiple Fibre Channel switches, subject to perfor-mance limitations. A second SMA could be added for redundancy, but it would have to remain inactive until needed.
Asymmetrical virtualisation avoids this problem. It allocates storage blocks to the servers, which must then store the block map needed to keep track of these allocations. This could be done in software, but for performance reasons, and to avoid changes to applications, a modified HBA card is the preferred route.
StoreAge, for example, has developed a SAN appliance to support virtual volume management across heterogeneous storage. It relies upon StoreAge's own PCI-based intelligent HBAs, and allows users and applications to request services from the SMA, which the firm calls a Metadata Centre, because it creates and distributes metadata:Ã, data about data.
The StoreAge technology is proprietary, but similar open technologies are under development. The best example is probably Compaq's VersaStor technology, which has attracted support from key SAN companies including StoreAge and IBM, as well as Fibre Channel switch developers Brocade and McData, and HBA developers such as Emulex, JNI and QLogic.
VersaStor, too, will rely on block maps loaded into HBAs by a storage management appliance, which in turn has to be intelligent enough to handle load distribution and data migration. This still leaves the SMA as a potential single point of failure, but if it does fail the SAN can continue as normal, using the stored block maps.
Symmetrical schemes have the advantage that they can work with today's SAN hardware, whereas VersaStor must await the development of proprietary appliances. But asymmetrical virtualisation may offer higher reliability and performance.
