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diff --git a/Documentation/filesystems/pohmelfs/design_notes.txt b/Documentation/filesystems/pohmelfs/design_notes.txt deleted file mode 100644 index 106d17fbb05f..000000000000 --- a/Documentation/filesystems/pohmelfs/design_notes.txt +++ /dev/null @@ -1,72 +0,0 @@ -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 authentication 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 according to IO priorities. -Administrators can add or remove servers from the set at run-time via special commands (described -in Documentation/filesystems/pohmelfs/info.txt file). Writes are replicated to all servers, which -are connected with write permission turned on. IO priority and permissions can be changed in -run-time. - -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. |