doc: Add an overview page for the project

Change-Id: I21b793c27aa7f24ffa1235c4a2c43d594148a689
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.gerrithub.io/392982
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>

doc/Doxyfile

@@ -782,6 +782,7 @@ WARN_LOGFILE           =
 
 INPUT                  = ../include/spdk \
                          index.md \
+                         about.md \
                          changelog.md \
                          directory_structure.md \
                          getting_started.md \

doc/about.md

@@ -0,0 +1,36 @@
+# What is SPDK? {#about}
+
+The Storage Performance Development Kit (SPDK) provides a set of tools and
+libraries for writing high performance, scalable, user-mode storage
+applications. It achieves high performance through the use of a number of key
+techniques:
+
+* Moving all of the necessary drivers into userspace, which avoids syscalls
+  and enables zero-copy access from the application.
+* Polling hardware for completions instead of relying on interrupts, which
+  lowers both total latency and latency variance.
+* Avoiding all locks in the I/O path, instead relying on message passing.
+
+The bedrock of SPDK is a user space, polled-mode, asynchronous, lockless
+[NVMe](http://www.nvmexpress.org) driver. This provides zero-copy, highly
+parallel access directly to an SSD from a user space application. The driver is
+written as a C library with a single public header. See @ref nvme for more
+details.
+
+SPDK further provides a full block stack as a user space library that performs
+many of the same operations as a block stack in an operating system. This
+includes unifying the interface between disparate storage devices, queueing to
+handle conditions such as out of memory or I/O hangs, and logical volume
+management. See @ref bdev for more information.
+
+Finally, SPDK provides
+[NVMe-oF](http://www.nvmexpress.org/nvm-express-over-fabrics-specification-released),
+[iSCSI](https://en.wikipedia.org/wiki/ISCSI), and
+[vhost](http://blog.vmsplice.net/2011/09/qemu-internals-vhost-architecture.html)
+servers built on top of these components that are capable of serving disks over
+the network or to other processes. The standard Linux kernel initiators for
+NVMe-oF and iSCSI interoperate with these targets, as well as QEMU with vhost.
+These servers can be up to an order of magnitude more CPU efficient than other
+implementations. These targets can be used as examples of how to implement a
+high performance storage target, or used as the basis for production
+deployments.

doc/index.md

@@ -2,6 +2,7 @@
 
 # Introduction {#intro}
 
+- @ref about
 - @ref getting_started
 - @ref vagrant
 - @ref changelog