Staging: pohmelfs: documentation.

This patch includes POHMELFS design and implementation description.
Separate file includes mount options, default parameters and usage examples.

Signed-off-by: Eveniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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+POHMELFS: Parallel Optimized Host Message Exchange Layered File System.
+
+		Evgeniy Polyakov <zbr@ioremap.net>
+
+Homepage: http://www.ioremap.net/projects/pohmelfs
+
+POHMELFS first began as a network filesystem with coherent local data and
+metadata caches but is now evolving into a parallel distributed filesystem.
+
+Main features of this FS include:
+ * Locally coherent cache for data and metadata with (potentially) byte-range locks.
+	Since all Linux filesystems lock the whole inode during writing, algorithm
+	is very simple and does not use byte-ranges, although they are sent in
+	locking messages.
+ * Completely async processing of all events except creation of hard and symbolic
+	links, and rename events.
+	Object creation and data reading and writing are processed asynchronously.
+ * Flexible object architecture optimized for network processing.
+	Ability to create long paths to objects and remove arbitrarily huge
+	directories with a single network command.
+	(like removing the whole kernel tree via a single network command).
+ * Very high performance.
+ * Fast and scalable multithreaded userspace server. Being in userspace it works
+	with any underlying filesystem and still is much faster than async in-kernel NFS one.
+ * Client is able to switch between different servers (if one goes down, client
+	automatically reconnects to second and so on).
+ * Transactions support. Full failover for all operations.
+	Resending transactions to different servers on timeout or error.
+ * Read request (data read, directory listing, lookup requests) balancing between multiple servers.
+ * Write requests are replicated to multiple servers and completed only when all of them are acked.
+ * Ability to add and/or remove servers from the working set at run-time.
+ * Strong authentification and possible data encryption in network channel.
+ * Extended attributes support.
+
+POHMELFS is based on transactions, which are potentially long-standing objects that live
+in the client's memory. Each transaction contains all the information needed to process a given
+command (or set of commands, which is frequently used during data writing: single transactions
+can contain creation and data writing commands). Transactions are committed by all the servers
+to which they are sent and, in case of failures, are eventually resent or dropped with an error.
+For example, reading will return an error if no servers are available.
+
+POHMELFS uses a asynchronous approach to data processing. Courtesy of transactions, it is
+possible to detach replies from requests and, if the command requires data to be received, the
+caller sleeps waiting for it. Thus, it is possible to issue multiple read commands to different
+servers and async threads will pick up replies in parallel, find appropriate transactions in the
+system and put the data where it belongs (like the page or inode cache).
+
+The main feature of POHMELFS is writeback data and the metadata cache.
+Only a few non-performance critical operations use the write-through cache and
+are synchronous: hard and symbolic link creation, and object rename. Creation,
+removal of objects and data writing are asynchronous and are sent to
+the server during system writeback. Only one writer at a time is allowed for any
+given inode, which is guarded by an appropriate locking protocol.
+Because of this feature, POHMELFS is extremely fast at metadata intensive
+workloads and can fully utilize the bandwidth to the servers when doing bulk
+data transfers.
+
+POHMELFS clients operate with a working set of servers and are capable of balancing read-only
+operations (like lookups or directory listings) between them.
+Administrators can add or remove servers from the set at run-time via special commands (described
+in Documentation/pohmelfs/info.txt file). Writes are replicated to all servers.
+
+POHMELFS is capable of full data channel encryption and/or strong crypto hashing.
+One can select any kernel supported cipher, encryption mode, hash type and operation mode
+(hmac or digest). It is also possible to use both or neither (default). Crypto configuration
+is checked during mount time and, if the server does not support it, appropriate capabilities
+will be disabled or mount will fail (if 'crypto_fail_unsupported' mount option is specified).
+Crypto performance heavily depends on the number of crypto threads, which asynchronously perform
+crypto operations and send the resulting data to server or submit it up the stack. This number
+can be controlled via a mount option.