Architecture

Figure 6: The MimdRAID architecture. Shaded parts are the modules added for an EW-Array.

The EW-Array prototype is implemented on the ``MimdRAID'' system developed by Yu et al [30]. Figure 6 shows how some of the MimdRAID modules have been replaced by EW-Array-specific components and how these components fit together. MimdRAID provides a framework and a number of useful features that enable one to conveniently experiment with novel disk array designs. We briefly highlight some of these useful features:

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MimdRAID exports a transparent logical disk interface on Windows 2000 so that the existing operating system and applications run unmodified on the underlying experimental disk array.

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Highly modularized components of MimdRAID, such as the ``Array Configuration'' and ``Scheduling'' components of Figure 6, can be replaced to allow for experimentation with new array designs.

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An accurate software-based disk head position prediction mechanism (in the ``Calibration'' layer) is crucial for realizing an EW-Array because the efficient scheduling of both eager-writes and reads depends on the precise knowledge of the rotational position of the disk head.

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At the lowest layer, the ``SCSI Abstraction'' module, which manages real Seagate disks, can be substituted with a disk simulator, so an EW-Array simulator and its implementation effectively share most of the code. The simulator can shorten simulation time of long traces by replacing physical I/O time with simulated time; it also allows us to experiment with a wide range of configurations without having to physically construct them.

Almost all the EW-Array-specific code in MimdRAID is concentrated in the Scheduling and Array Configuration layers. The Scheduling layer implements all the eager-writing scheduling algorithms described in Section 4. The Array Configuration layer is responsible for translating requests of logical I/O addresses to those of physical I/O addresses. This layer maintains a logical-to-physical address mapping and we describe this next.