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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>
This commit is contained in:
Evgeniy Polyakov 2009-02-09 17:02:34 +03:00 committed by Greg Kroah-Hartman
parent e333720166
commit b8523c40d5
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Documentation/filesystems/pohmelfs/design_notes.txt Normal file
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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.

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POHMELFS usage information.
Mount options:
idx=%u
Each mountpoint is associated with a special index via this option.
Administrator can add or remove servers from the given index, so all mounts,
which were attached to it, are updated.
Default it is 0.
trans_scan_timeout=%u
This timeout, expressed in milliseconds, specifies time to scan transaction
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
the system checks that servers have 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 an error.
Default is 5 seconds.
trans_retries=%u
This is the number of times that a transaction will be resent to a server that did
not answer for the last @trans_scan_timeout milliseconds.
When the number of resends exceeds this limit, the transaction is completed with error.
Default is 5 resends.
crypto_thread_num=%u
Number of crypto processing threads. Threads are used both for RX and TX traffic.
Default is 2, or no threads if crypto operations are not supported.
trans_max_pages=%u
Maximum number of pages in a single transaction. This parameter also controls
the number of pages, allocated for crypto processing (each crypto thread has
pool of pages, the number of which is equal to 'trans_max_pages'.
Default is 100 pages.
crypto_fail_unsupported
If specified, mount will fail if the server does not support requested crypto operations.
By default mount will disable non-matching crypto operations.
mcache_timeout=%u
Maximum number of milliseconds to wait for the mcache objects to be processed.
Mcache includes locks (given lock should be granted by server), attributes (they should be
fully received in the given timeframe).
Default is 5 seconds.
Usage examples.
Add (or remove if it already exists) server server1.net:1025 into the working set with index $idx
with appropriate hash algorithm and key file and cipher algorithm, mode and key file:
$cfg -a server1.net -p 1025 -i $idx -K $hash_key -k $cipher_key
Mount filesystem with given index $idx to /mnt mountpoint.
Client will connect to all servers specified in the working set via previous command:
mount -t pohmel -o idx=$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, server will negotiate hash/cipher with client, in case client requested it, there
are appropriate key files.
Number of working threads is set to 10.
# ./fserver -a 0.0.0.0 -p 1025 -r /mnt -w 10 -K hash_key -k cipher_key
-A 6 - listen on ipv6 address. Default: Disabled.
-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.
-K file - hash key size. Default: none.
-k file - cipher key size. Default: none.
-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).

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POHMELFS network protocol.
Basic structure used in network communication is following command:
struct netfs_cmd
{
__u16 cmd; /* Command number */
__u16 csize; /* Attached crypto information size */
__u16 cpad; /* Attached padding size */
__u16 ext; /* External flags */
__u32 size; /* Size of the attached data */
__u32 trans; /* Transaction id */
__u64 id; /* Object ID to operate on. Used for feedback.*/
__u64 start; /* Start of the object. */
__u64 iv; /* IV sequence */
__u8 data[0];
};
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.
All commans are transfered over the network in big-endian. CPU endianess is used at the end peers.
@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_PAGE_CACHE, /* Page cache invalidation message */
NETFS_READ_PAGES, /* Read multiple contiguous pages in one go */
NETFS_RENAME, /* Rename object */
NETFS_CAPABILITIES, /* Capabilities of the client, for example supported crypto */
NETFS_LOCK, /* Distributed lock message */
NETFS_XATTR_SET, /* Set extended attribute */
NETFS_XATTR_GET, /* Get extended attribute */
@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.
@csize, @cpad - size and padding size of the (attached if needed) crypto information.
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_PAGE_CACHE
Command is only received by clients. It contains information about
page to be marked as not up-to-date.
@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.
@NETFS_CAPABILITIES
Used to exchange crypto capabilities with server.
If crypto capabilities are not supported by server, then client will disable it
or fail (if 'crypto_fail_unsupported' mount options was specified).
@id - superblock index. Used to specify crypto information for group of servers.
@size - size of the attached capabilities structure.
@start - 0.
@size - 0.
@scsize - 0.
@NETFS_LOCK
Used to send lock request/release messages. Although it sends byte range request
and is capable of flushing pages based on that, it is not used, since all Linux
filesystems lock the whole inode.
@id - lock generation number.
@start - start of the locked range.
@size - size of the locked range.
@ext - lock type: read/write. Not used actually. 15'th bit is used to determine,
if it is lock request (1) or release (0).
@NETFS_XATTR_SET
@NETFS_XATTR_GET
Used to set/get extended attributes for given inode.
@id - attribute generation number or xattr setting type
@start - size of the attribute (request or attached)
@size - name length, path len and data size for given attribute
@ext - path length for given object