The Basic Interface

Current disks support the ability to write data of a specified length to a given address on disk. With range writes, the disk supports the ability to write data to one address out of a set of specified options. The options are specified in a range descriptor. The simplest possible range descriptor is comprised of a pair of addresses, B_begin and B_end, designating that data of a given length can be written to any contiguous range of blocks fitting within B_begin and B_end. See Figure 1 for an example.

Figure: Range Writes. The figure illustrates how to use range writes. The disk has just serviced a write to block 5, and is then given a write with two ranges: 13 through 16, and 19 through 22. The disk, given its current position, realizes that 19 will result in the fastest write, and thus chooses it as the target. The file system is subsequently informed.

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When the operation completes, the disk returns a target address, that is, the starting address of the region to which it wrote the data. This information allows the file system to record the address of the data block in whatever structure it is using (e.g., an inode).

One option the client must include is the alignment of writes, which restricts where in the range a write can be placed. For example, if the file system is writing a 4-KB block to a 16-KB range, it would specify an alignment of 4 KB, thus restricting the write to one of four locations. Without such an option, the disk could logically choose to start writing at any 512-byte offset within the range.

The interface also guarantees no reordering of blocks within a single multi-block write. This decision enables the requester to control the ordering of blocks on disk, which may be important for subsequent read performance, and allows a single target address to be returned by the disk. The interface is summarized in Table 1.