Asynchronous I/O moves forward

There has been little (visible) progress with the asynchronous I/O code since the AIO core was merged into the 2.5.32 kernel. AIO author Ben LaHaise has not been idle, however. Slowly the other pieces of the AIO package are beginning to show up for the 2.5 tree.

One piece is this patch which adds "synchronous IOCBs" to the mix. One might wonder why an asynchronous I/O infrastructure needs I/O control blocks which have a synchronous option. The answer is that the synchronous IOCB is needed to achieve the goal of making most or all low-level I/O operations in the kernel be asynchronous. Once the I/O primitives expect an IOCB, and they work in an asynchronous mode, it is easy to layer the older, synchronous versions on top through the use of a synchronous IOCB. For now, synchronous IOCBs are only used in the generic_file_read() function.

The next step, perhaps, is this patch from Badari Pulavarty; which reworks the direct I/O (DIO) infrastructure. The DIO code handles direct operations on block devices - such as when a "raw" device is used, or when a file is opened with the O_DIRECT option. The DIO operations, with this patch, are all asynchronous, with synchronous IOCBs used when synchronous behavior is required. With this change, the task of making the block I/O subsystem be asynchronous internally is nearly complete. Other subsystems (i.e. char devices, networking) remain to be converted over to the AIO scheme, however.

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