Monday, October 10, 2011

Intelligent Disks

We will now look at yet another proposal from researchers at the University of California at Berkeley, a place with a high output in the field of innovative architectures. Their proposal is to do with making good use of the processing capabilities embedded with each disk drive; they call it Intelligent Disks (or IDISKs).

In the early days of magnetic peripherals, the interface provided facilities closely matched to the operations the disk mechanism could naturally perform. This changed; in the 1960s, with processing power causing a system bottleneck, disk units with autonomous data-handling capabilities began to appear, with a concomitant increase in the level of abstraction supported by their interfaces. An example from IBM in the S/360 family was the ckd interface, which, for example, allowed the system to delegate to the disk a key-based search for a record.

The economics of minicomputers and then microprocessors made processing power cheap enough that intelligence began to migrate back into the system proper, and we saw the emergence of standard low-level disk interfaces like SMD and ESDI. The growth of complexity in the peripherals drove the existence of a higher-level interface, SCSI. Implementing SCSI more or less requires a microprocessor and supporting memory, to provide protocol support, cache, local management of the peripheral, management of read/write head positioning, error management, buffer space for SCSI bus transfers and so on. Current technology already makes it possible to integrate, low cost, and low power processing/memory and interface electronics directly with the peripherals themselves, along with system interface support (such as FC-AL). The idea behind intelligent disks is to make greater use of this embedded intelligence, allowing both off-loading of the server and greater computational concurrency. We note here the balance swinging back: new technology allows us to effectively re-implement old ideas.

Various levels of functionality may be envisaged for intelligent disks.; Examples include:

» Off-load database management software, along with necessary supporting OS functions, in a share-nothing architecture implemented across the disks themselves

» Off-load some database management functions onto the disks—for example, searches and joins, with the control of these operations being done by the server-resident DBMS

» Off-load file system activities—for example, optimizing a string of commands by reordering them in the disk to minimize head movement

While there is little quantitative information available for the benefits this approach can bring, the real problem is standardization. Indeed, widespread deployment of intelligent disks would mean that the most widely used software (from basic file systems with simple access methods, such as those offered by Windows and Linux, all the way up to DBMSs) would be adapted to such machinery. This requires agreements between the players behind this software and the disk manufacturers; an agreement between the disk manufacturers is in the realm of the possible, and is in any case a prerequisite for any larger agreement between software vendors and the disk manufacturers.

These latter possibility seems more remote; we can easily observe that historically, DBMS manufacturers have avoid using the magic features of any system platform, preferring to use extremely low-level interfaces so that they could be certain that execution of vital functionalities is under their control and could therefore be guaranteed across platforms. This view also simplifies porting across platforms and reduces support and development costs for the DBMS.

Source of Information : Elsevier Server Architectures 2005
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