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    Subject[2/3] POHMELFS: Documentation.
    Design notes, usage cases and protocol description.

    Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>

    diff --git a/Documentation/filesystems/pohmelfs/design_notes.txt b/Documentation/filesystems/pohmelfs/design_notes.txt
    new file mode 100644
    index 0000000..c9a9379
    --- /dev/null
    +++ b/Documentation/filesystems/pohmelfs/design_notes.txt
    @@ -0,0 +1,61 @@
    +POHMELFS: Parallel Optimized Host Message Exchange Layered File System.
    +
    + Evgeniy Polyakov <johnpol@2ka.mipt.ru>
    +
    +Homepage: http://tservice.net.ru/~s0mbre/old/?section=projects&item=pohmelfs
    +
    +It was first started as network filesystem with coherent local data and metadata caches,
    +but it is being evolved into parallel distibuted filesystem now.
    +
    +Main features of this FS include:
    + * Local coherent (notes cache for data and metadata:
    + http://tservice.net.ru/~s0mbre/blog/devel/fs/2008_05_17.html)
    + * Completely async processing of all events (hard, symlinks and rename are the
    + only exceptions) including object creation and data reading and writing.
    + * Flexible object architecture optimized for network processing.
    + Ability to create long pathes to object and remove arbitrary huge
    + directoris in single network command.
    + (like removing the whole kernel tree via 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 requests (data read, directory listing, lookup requests) balancing between multiple servers.
    + * Write requests are sent to multiple servers and completed only when all of them sent an ack.
    + * Ability to add and/or remove servers from working set at run-time from userspace (via netlink,
    + so the same command can be processed from real network though, but since server does
    + not support it yet, I dropped network part).
    +
    +
    +POHMELFS is based on transactions, which are potentially long-standing objects, which live
    +in clients memory. Each transaction contains all information needed to process given command
    +(or set of command, which is frequently used during data writing: single transaction can contain
    +creation and data writing commands). Transaction is committed by all servers, where it was sent,
    +and in case of failure of one or another, it will be eventually resent or dropped with error,
    +so, for example, reading will return error, if no servers are available.
    +
    +POHMELFS uses novel asynchronous approach of data processing. Courtesy to transactions, it is
    +possible to detouch reply from request, and if command requires data to be received, caller
    +just sleeps waiting for it. Thus it is possible to issue multiple read commands to different
    +servers and async threads will pick replies in parallel, find appropriate transactions in the
    +system and put data where it belongs (like page or inode cache).
    +
    +Main feature of the POHMELFS is writeback data and metadata cache.
    +Only few non-performance critical operations use write-through cache and are synchronous:
    +hard and symbolic link creation and object rename. Creation and removal of objects,
    +as long as writing, are asynchronous and are sent to the server during system writeback.
    +When server receives some request for given object in the system (like data reading,
    +or file creation or whatever else), it stores appropriate client information in own cache,
    +so when subsequent request comes from different client, all previous could be notified
    +(for example when several clients read data from file, and then new client writes there,
    +appropriate pages on clients will be invalidated, so subsequent write will force them
    +to read page from the server). Because of this feature POHMELFS is extremely fast in metadata
    +intensive workloads, and can fully utilize bandwidth to servers when doing bulk data transafers.
    +
    +POHMELFS client operates with working set of servers, and is capable of balancing reading,
    +i.e. no modification, like lookup or directory listing, workload between all servers it was conneted to.
    +Administrator can add or remove servers from the set in run-time via special command (described
    +in Documentation/pohmelfs/info.txt file). Writing is performed to all servers.
    diff --git a/Documentation/filesystems/pohmelfs/info.txt b/Documentation/filesystems/pohmelfs/info.txt
    new file mode 100644
    index 0000000..1b4a3e3
    --- /dev/null
    +++ b/Documentation/filesystems/pohmelfs/info.txt
    @@ -0,0 +1,61 @@
    +POHMELFS usage information.
    +
    +Mount options:
    +idx=%u
    + Each mountpoint is associated with special index via this option.
    + Administrator can add or remove servers from given index, so all mounts,
    + which were attached to it, were updated.
    + Default it is 0.
    +
    +trans_scan_timeout=%u
    + This timeout, expressed in milliseconds, specifies time to scan trasaction
    + trees looking for stale requests, which have to be resent, or if number of
    + retries exceed specified limit, dropped with error.
    + Default is 5 seconds.
    +
    +drop_scan_timeout=%u
    + Internal timeout, expressed in milliseconds, which specifies how frequently
    + inodes marked to be dropped are freed. It also specifies how frequently
    + system checks, that servers has to be added or removed from current working set.
    + Default is 1 second.
    +
    +wait_on_page_timeout=%u
    + Number of milliseconds to wait for reply from remote server for data reading command.
    + If this timeout is exceeded, reading returns error.
    + Default is 5 seconds.
    +
    +trans_retries=%u
    + Number of times, transaction will be resent to the server, which did not answer for the
    + last @trans_scan_timeout milliseconds. When number of resends exceeds this limit,
    + transaction is completed with error.
    + Default is 5 resends.
    +
    +
    +Usage examples.
    +
    +Add (or remove if it already exists) server server1.net:1025 into working set with index $idx:
    +$cfg -a server1.net -p 1025 -i $idx
    +
    +Mount filesystem with given index $idx to /mnt mountpoint.
    +Client will connect to all servers specified in working set via previous command:
    +mount -t pohmel -o idx=$idx q /mnt
    +
    +One can add or remove servers from working set after mounting too.
    +
    +
    +Server installation.
    +
    +Creating a server, which listens at port 1025 and 0.0.0.0 address.
    +Working root directory (note, that server chroots there, so you have to have appropriate permissions)
    +is set to /mnt. Number of working threads is set to 10.
    +
    +# ./fserver -a 0.0.0.0 -p 1025 -r /mnt -w 10
    +
    + -r root - path to root directory. Default: /tmp.
    + -a addr - listen address. Default: 0.0.0.0.
    + -p port - listen port. Default: 1025.
    + -w workers - number of workers per connected client. Default: 1.
    + -h - this help.
    +
    +Number of worker threads specifies how many workers will be created for each client.
    +Bulk single-client transafers usually are better handled with smaller number (like 1-3).
    diff --git a/Documentation/filesystems/pohmelfs/network_protocol.txt b/Documentation/filesystems/pohmelfs/network_protocol.txt
    new file mode 100644
    index 0000000..f14910a
    --- /dev/null
    +++ b/Documentation/filesystems/pohmelfs/network_protocol.txt
    @@ -0,0 +1,202 @@
    +POHMELFS network protocol.
    +
    +Basic structure used in network communication is following command:
    +
    +struct netfs_cmd
    +{
    + __u16 cmd; /* Command number */
    + __u16 ext; /* External flags */
    + __u32 size; /* Size of the attached data */
    + __u64 id; /* Object ID to operate on. Used for feedback.*/
    + __u64 start; /* Start of the object. */
    + __u8 data[];
    +};
    +
    +Commands can be embedded into transaction command (which in turn has own command),
    +so one can extend protocol as needed without breaking backward compatibility as long
    +as old commands are supported. All string lengths include tail 0 byte.
    +
    +@cmd - command number, which specifies command to be processed. Following
    + commands are used currently:
    +
    + NETFS_READDIR = 1, /* Read directory for given inode number */
    + NETFS_READ_PAGE, /* Read data page from the server */
    + NETFS_WRITE_PAGE, /* Write data page to the server */
    + NETFS_CREATE, /* Create directory entry */
    + NETFS_REMOVE, /* Remove directory entry */
    + NETFS_LOOKUP, /* Lookup single object */
    + NETFS_LINK, /* Create a link */
    + NETFS_TRANS, /* Transaction */
    + NETFS_OPEN, /* Open intent */
    + NETFS_INODE_INFO, /* Metadata cache coherency synchronization message */
    + NETFS_JOIN_GROUP, /* Joing metadata update group */
    + NETFS_LEAVE_GROUP, /* Leave metadata update group */
    + NETFS_PAGE_CACHE, /* Page cache invalidation message */
    + NETFS_READ_PAGES, /* Read multiple contiguous pages in one go */
    + NETFS_RENAME, /* Rename object */
    +
    +@ext - external flags. Used by different commands to specify some extra arguments
    + like partial size of the embedded objects or creation flags.
    +
    +@size - size of the attached data. For NETFS_READ_PAGE and NETFS_READ_PAGES no data is attached,
    + but size of the requested data is incorporated here. It does not include size of the command
    + header (struct netfs_cmd) itself.
    +
    +@id - id of the object this command operates on. Each command can use it for own purpose.
    +
    +@start - start of the object this command operates on. Each command can use it for own purpose.
    +
    +
    +Command specifications.
    +
    +@NETFS_READDIR
    +This command is used to sync content of the remote dir to the client.
    +
    +@ext - length of the path to object.
    +@size - the same.
    +@id - local inode number of the directory to read.
    +@start - zero.
    +
    +
    +@NETFS_READ_PAGE
    +This command is used to read data from remote server.
    +Data size does not exceed local page cache size.
    +
    +@id - inode number.
    +@start - first byte offset.
    +@size - number of bytes to read plus length of the path to object.
    +@ext - object path length.
    +
    +
    +@NETFS_CREATE
    +Used to create object.
    +It does not require that all directories on top of the object were
    +already created, it will create them automatically. Each object has
    +associated @netfs_path_entry data structure, which contains creation
    +mode (permissions and type) and length of the name as long as name itself.
    +
    +@start - 0
    +@size - size of the all data structures needed to create a path
    +@id - local inode number
    +@ext - 0
    +
    +
    +@NETFS_REMOVE
    +Used to remove object.
    +
    +@ext - length of the path to object.
    +@size - the same.
    +@id - local inode number.
    +@start - zero.
    +
    +
    +@NETFS_LOOKUP
    +Lookup information about object on server.
    +
    +@ext - length of the path to object.
    +@size - the same.
    +@id - local inode number of the directory to look object in.
    +@start - local inode number of the object to look at.
    +
    +
    +@NETFS_LINK
    +Create hard of symlink.
    +Command is sent as "object_path|target_path".
    +
    +@size - size of the above string.
    +@id - parent local inode number.
    +@start - 1 for symlink, 0 for hardlink.
    +@ext - size of the "object_path" above.
    +
    +
    +@NETFS_TRANS
    +Transaction header.
    +
    +@size - incorporates all embedded command sizes including theirs header sizes.
    +@start - transaction generation number - unique id used to find transaction.
    +@ext - transaction flags. Unused at the moment.
    +@id - 0.
    +
    +
    +@NETFS_OPEN
    +Open intent for given transaction.
    +
    +@id - local inode number.
    +@start - 0.
    +@size - path length to the object.
    +@ext - open flags (O_RDWR and so on).
    +
    +
    +@NETFS_INODE_INFO
    +Metadata update command.
    +It is sent to servers when attributes of the object are changed and received
    +when data or metadata were updated. It operates with the following structure:
    +
    +struct netfs_inode_info
    +{
    + unsigned int mode;
    + unsigned int nlink;
    + unsigned int uid;
    + unsigned int gid;
    + unsigned int blocksize;
    + unsigned int padding;
    + __u64 ino;
    + __u64 blocks;
    + __u64 rdev;
    + __u64 size;
    + __u64 version;
    +};
    +
    +It effectively mirrors stat(2) returned data.
    +
    +
    +@ext - path length to the object.
    +@size - the same plus size of the netfs_inode_info structure.
    +@id - local inode number.
    +@start - 0.
    +
    +
    +@NETFS_JOIN_GROUP/NETFS_LEAVE_GROUP
    +Metadata cache coherency synchronization messages.
    +They are broadcasted when new inode is created (either for new object
    +or object read from the server), so that server new inode number of the
    +object on the appropriate client. @NETFS_LEAVE_GROUP is sent when local
    +inode is destroyed, so that client physically can not be interested in
    +data or metadata updates for given inode.
    +
    +@ext - path length to the object.
    +@size - the same.
    +@id - local inode number for given object.
    +@start - 0.
    +
    +
    +@NETFS_PAGE_CACHE
    +Command is only received by clients. It contains information about
    +page to be marked as not up-to-date. Server fills this command with data,
    +provided by above @NETFS_JOIN_GROUP command.
    +
    +@id - client's inode number.
    +@start - last byte of the page to be invalidated. If it is not equal to
    + current inode size, it will be vmtruncated().
    +@size - 0
    +@ext - 0
    +
    +
    +@NETFS_READ_PAGES
    +Used to read multiple contiguous pages in one go.
    +
    +@start - first byte of the contiguous region to read.
    +@size - contains of two fields: lower 8 bits are used to represent page cache shift
    + used by client, another 3 bytes are used to get number of pages.
    +@id - local inode number.
    +@ext - path length to the object.
    +
    +
    +@NETFS_RENAME
    +Used to rename object.
    +Attached data is formed into following string: "old_path|new_path".
    +
    +@id - local inode number.
    +@start - parent inode number.
    +@size - length of the above string.
    +@ext - length of the old path part.


    --
    Evgeniy Polyakov


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