Virtualization in the switch? Not so fast


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Mirroring and striping
Mirroring is one of the most simple, yet most powerful virtualization techniques. The latency injected by the virtualization operator in mirroring is minimal as there's no parsing of the command and no block address manipulation to do. Additionally, error recovery is simplest for mirroring compared with other forms of storage virtualization.

Striping takes advantage of parallel operation on storage to provide faster performance. Incoming I/O operations from initiators would be terminated by the virtualization operator and a number of reinitiated I/O operations would be created, corresponding to the number of downstream storage IDs in the virtual device.

Striping requires that the incoming I/O operation be split up among the member storage destinations of the virtual device. While striping without parity redundancy is used solely for performance purposes, it's not clear that using striping in a switch would result in a net performance gain, after the latency of the virtualization operator is taken into account. Also, considering the lack of protection from failures, it's likely to be used only in cases where flexibility in creating striped arrays of different sizes is the highest priority.

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Reinitiating an I/O command

The terminal and reinstallation process.

RAID in a switch uses parity or mirrored redundancy to provide data protection from downstream storage and link failures. As with striping, the incoming I/O would be terminated by the virtualization operator, and the number of subsequent downstream I/Os would be determined by the number of member storage IDs in the array.

Parity RAID adds a significant level of overhead to the virtualization operator. For example, the read, modify and write penalty of RAID-5 would be a much higher burden on a switch processor than mirrored redundancy such as used in RAID-1 or 0+1. As RAID-1 was previously discussed as mirroring earlier, RAID-0+1 should be the RAID level of choice for switch virtual storage implementations to minimize the negative impacts of unanticipated write and switch workloads.

Concatenation merges two separate storage address spaces together and utilizes them as a single, virtual contiguous storage address space. Like striping, it has no built-in redundancy to protect against downstream link and storage failures. Unlike striping, however, a failure in a concatenated virtual device doesn't necessarily result in the loss of all data because data that's stored completely on any downstream storage IDs may still be able to be accessed. However, there's no guarantee that data objects were written completely on any one device of a concatenated storage volume. The overhead of concatenation is relatively minor, involving simple algebraic processes for translating storage addresses.

Fierce competition
In the heyday of the technology bubble when everything seemed possible, storage switch vendors expected the market to gobble up virtualization switches like hotcakes. Times have certainly changed and the prospects of switch-based virtualization don't seem like such a sure thing anymore.

The issue for IT professionals is whether or not switch vendors will provide virtualization functions that can add enough value to make it worthwhile to pay a premium for it. This can only happen by making storage easier to manage and less expensive to own. Already, volume management software and disk subsystem controllers provide complete functionality for the most useful types of storage virtualization.

Beyond competition with volume management software and disk subsystems, there are new virtualization products and architectures to contend with. Switch-based virtualization is another instance of in-band virtualization, a concept that hasn't been too warmly received by the market yet. If the market decides that out-of-band virtualization is a better way to go as Gartner Inc. has suggested, that would leave the switch virtualization players holding a mostly empty bag.

This was first published in February 2003

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