72 lines
4.5 KiB
Plaintext
72 lines
4.5 KiB
Plaintext
|
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 according to IO priorities.
|
||
|
|
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, 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.
|