Raw Disk I/O

System-based I/O vs. Raw I/O
Before version 2.2, the Linux kernel supported only one type of disk
I/O -- system-based I/O in which a process interacts with a physical
device through an intermediary kernel buffer. This intermediary buffer
is transparent to the user: calls to read() write() lseek() etc., seem
as if they access a physical file directly. In practice, however, the
kernel intercepts the calls and transfers the data to its own buffer
before passing it on to the physical device or process. System-based
I/O offers several advantages. The kernel can cache data and thus
reduce the overall physical I/O operations needed; the kernel controls
the overall system performance because it won't allow an on-the-loose
process to starve other processes. Finally, certain devices are fussy
about the size of data being written or read (for instance, disk drives
usually handle fixed size chunks of 512 bytes on each I/O operation).
The kernel hides these physical limitations from the user. In raw disk
I/O, the process interacts with a physical device directly, without the
kernel's brokerage.

Uses of raw disk I/O
Although the traditional system-based I/O is satisfactory in most
cases, some applications must use raw I/O. One common scenario is in
data-critical applications, where the user wants to ensure that the
data is written to a disk immediately so that it isn't lost in the
event of a system failure. Specialized applications, say a relational
database engine, often use their own I/O caching algorithms. In such
applications, the overhead of the kernel's caching is uncalled for.

Raw I/O Past, Present, and Future
The concept of raw disk I/O is hardly new. Several attempts have been
made in the past to introduce raw I/O to Unix. Indeed, quite a few
variants such as IBM's AIX now support it. The problem, however, is
that most existing implementations require literally doubling the
number of device nodes. Linux creators rejected this approach. Instead,
kernel 2.4 uses a pool of device nodes that can be associated with any
arbitrary block device. A new object called "kiobuf" was introduced. A
kiobuf is an abstraction of a set of kernel pages set up during boot
time. Raw I/O is achieved by creating a kiobuf and populating it with
the physical pages a given process is using for I/O, without any
intermediate copies.

The kiobuf object is then passed to the I/O layers for reading and
writing. The current Linux implementation isn't perfect yet but it's
constantly improving. For further information, you can subscribe to the
kiobuf-io-devel list server or read prior postings here:
http://lists.sourceforge.net/lists/listinfo/kiobuf-io-devel

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