- A little new content for the Architecture Manual describing

what happened to the traditional UNIX block devices.

Approved:	blackend (mentor)
Reviewed:	phk (he actually rewrote most of it...)

en_US.ISO8859-1/books/arch-handbook/driverbasics/chapter.sgml

@@ -527,6 +527,33 @@ Test Data</screen>
     </para>
   </sect1>
 
+  <sect1 id="driverbasics-block">
+    <title>Block Devices (Are Gone)</title>
+
+    <para>Other &unix; systems may support a second type of disk
+      device known as block devices.  Block devices are disk devices
+      for which the kernel provides caching.  This caching makes
+      block-devices almost unusable, or at least dangerously
+      unreliable.  The caching will reorder the sequence of write
+      operations, depriving the application of the ability to know
+      the exact disk contents at any one instant in time.  This
+      makes predictable and reliable crash recovery of on-disk data
+      structures (filesystems, databases etc.) impossible.
+      Since writes may be delayed, there is no way the kernel can
+      report to the application which particular write operation
+      encountered a write error, this further compounds the
+      consistency problem.  For this reason, no serious applications
+      rely on block devices, and in fact, almost all applications
+      which access disks directly take great pains to specify that
+      character (or <quote>raw</quote>) devices should always be
+      used.  Because the implementation of the aliasing of each disk
+      (partition) to two devices with different semantics significantly
+      complicated the relevant kernel code &os; dropped support for
+      cached disk devices as part of the modernization of the disk I/O
+      infrastructure.
+    </para>
+  </sect1>
+
   <sect1 id="driverbasics-net">
     <title>Network Drivers</title>