io_uring: do not always copy iovec in io_req_map_rw()

[linux/fpc-iii.git] / Documentation / filesystems / orangefs.rst

.. SPDX-License-Identifier: GPL-2.0

========
ORANGEFS
========

OrangeFS is an LGPL userspace scale-out parallel storage system. It is ideal
for large storage problems faced by HPC, BigData, Streaming Video,
Genomics, Bioinformatics.

Orangefs, originally called PVFS, was first developed in 1993 by
Walt Ligon and Eric Blumer as a parallel file system for Parallel
Virtual Machine (PVM) as part of a NASA grant to study the I/O patterns
of parallel programs.

Orangefs features include:

  * Distributes file data among multiple file servers
  * Supports simultaneous access by multiple clients
  * Stores file data and metadata on servers using local file system
    and access methods
  * Userspace implementation is easy to install and maintain
  * Direct MPI support
  * Stateless


Mailing List Archives
=====================

http://lists.orangefs.org/pipermail/devel_lists.orangefs.org/


Mailing List Submissions
========================

devel@lists.orangefs.org


Documentation
=============

http://www.orangefs.org/documentation/


Userspace Filesystem Source
===========================

http://www.orangefs.org/download

Orangefs versions prior to 2.9.3 would not be compatible with the
upstream version of the kernel client.


Running ORANGEFS On a Single Server
===================================

OrangeFS is usually run in large installations with multiple servers and
clients, but a complete filesystem can be run on a single machine for
development and testing.

On Fedora, install orangefs and orangefs-server::

    dnf -y install orangefs orangefs-server

There is an example server configuration file in
/etc/orangefs/orangefs.conf.  Change localhost to your hostname if
necessary.

To generate a filesystem to run xfstests against, see below.

There is an example client configuration file in /etc/pvfs2tab.  It is a
single line.  Uncomment it and change the hostname if necessary.  This
controls clients which use libpvfs2.  This does not control the
pvfs2-client-core.

Create the filesystem::

    pvfs2-server -f /etc/orangefs/orangefs.conf

Start the server::

    systemctl start orangefs-server

Test the server::

    pvfs2-ping -m /pvfsmnt

Start the client.  The module must be compiled in or loaded before this
point::

    systemctl start orangefs-client

Mount the filesystem::

    mount -t pvfs2 tcp://localhost:3334/orangefs /pvfsmnt


Building ORANGEFS on a Single Server
====================================

Where OrangeFS cannot be installed from distribution packages, it may be
built from source.

You can omit --prefix if you don't care that things are sprinkled around
in /usr/local.  As of version 2.9.6, OrangeFS uses Berkeley DB by
default, we will probably be changing the default to LMDB soon.

::

    ./configure --prefix=/opt/ofs --with-db-backend=lmdb

    make

    make install

Create an orangefs config file::

    /opt/ofs/bin/pvfs2-genconfig /etc/pvfs2.conf

Create an /etc/pvfs2tab file::

    echo tcp://localhost:3334/orangefs /pvfsmnt pvfs2 defaults,noauto 0 0 > \
        /etc/pvfs2tab

Create the mount point you specified in the tab file if needed::

    mkdir /pvfsmnt

Bootstrap the server::

    /opt/ofs/sbin/pvfs2-server -f /etc/pvfs2.conf

Start the server::

    /opt/osf/sbin/pvfs2-server /etc/pvfs2.conf

Now the server should be running. Pvfs2-ls is a simple
test to verify that the server is running::

    /opt/ofs/bin/pvfs2-ls /pvfsmnt

If stuff seems to be working, load the kernel module and
turn on the client core::

    /opt/ofs/sbin/pvfs2-client -p /opt/osf/sbin/pvfs2-client-core

Mount your filesystem::

    mount -t pvfs2 tcp://localhost:3334/orangefs /pvfsmnt


Running xfstests
================

It is useful to use a scratch filesystem with xfstests.  This can be
done with only one server.

Make a second copy of the FileSystem section in the server configuration
file, which is /etc/orangefs/orangefs.conf.  Change the Name to scratch.
Change the ID to something other than the ID of the first FileSystem
section (2 is usually a good choice).

Then there are two FileSystem sections: orangefs and scratch.

This change should be made before creating the filesystem.

::

    pvfs2-server -f /etc/orangefs/orangefs.conf

To run xfstests, create /etc/xfsqa.config::

    TEST_DIR=/orangefs
    TEST_DEV=tcp://localhost:3334/orangefs
    SCRATCH_MNT=/scratch
    SCRATCH_DEV=tcp://localhost:3334/scratch

Then xfstests can be run::

    ./check -pvfs2


Options
=======

The following mount options are accepted:

  acl
    Allow the use of Access Control Lists on files and directories.

  intr
    Some operations between the kernel client and the user space
    filesystem can be interruptible, such as changes in debug levels
    and the setting of tunable parameters.

  local_lock
    Enable posix locking from the perspective of "this" kernel. The
    default file_operations lock action is to return ENOSYS. Posix
    locking kicks in if the filesystem is mounted with -o local_lock.
    Distributed locking is being worked on for the future.


Debugging
=========

If you want the debug (GOSSIP) statements in a particular
source file (inode.c for example) go to syslog::

  echo inode > /sys/kernel/debug/orangefs/kernel-debug

No debugging (the default)::

  echo none > /sys/kernel/debug/orangefs/kernel-debug

Debugging from several source files::

  echo inode,dir > /sys/kernel/debug/orangefs/kernel-debug

All debugging::

  echo all > /sys/kernel/debug/orangefs/kernel-debug

Get a list of all debugging keywords::

  cat /sys/kernel/debug/orangefs/debug-help


Protocol between Kernel Module and Userspace
============================================

Orangefs is a user space filesystem and an associated kernel module.
We'll just refer to the user space part of Orangefs as "userspace"
from here on out. Orangefs descends from PVFS, and userspace code
still uses PVFS for function and variable names. Userspace typedefs
many of the important structures. Function and variable names in
the kernel module have been transitioned to "orangefs", and The Linux
Coding Style avoids typedefs, so kernel module structures that
correspond to userspace structures are not typedefed.

The kernel module implements a pseudo device that userspace
can read from and write to. Userspace can also manipulate the
kernel module through the pseudo device with ioctl.

The Bufmap
----------

At startup userspace allocates two page-size-aligned (posix_memalign)
mlocked memory buffers, one is used for IO and one is used for readdir
operations. The IO buffer is 41943040 bytes and the readdir buffer is
4194304 bytes. Each buffer contains logical chunks, or partitions, and
a pointer to each buffer is added to its own PVFS_dev_map_desc structure
which also describes its total size, as well as the size and number of
the partitions.

A pointer to the IO buffer's PVFS_dev_map_desc structure is sent to a
mapping routine in the kernel module with an ioctl. The structure is
copied from user space to kernel space with copy_from_user and is used
to initialize the kernel module's "bufmap" (struct orangefs_bufmap), which
then contains:

  * refcnt
    - a reference counter
  * desc_size - PVFS2_BUFMAP_DEFAULT_DESC_SIZE (4194304) - the IO buffer's
    partition size, which represents the filesystem's block size and
    is used for s_blocksize in super blocks.
  * desc_count - PVFS2_BUFMAP_DEFAULT_DESC_COUNT (10) - the number of
    partitions in the IO buffer.
  * desc_shift - log2(desc_size), used for s_blocksize_bits in super blocks.
  * total_size - the total size of the IO buffer.
  * page_count - the number of 4096 byte pages in the IO buffer.
  * page_array - a pointer to ``page_count * (sizeof(struct page*))`` bytes
    of kcalloced memory. This memory is used as an array of pointers
    to each of the pages in the IO buffer through a call to get_user_pages.
  * desc_array - a pointer to ``desc_count * (sizeof(struct orangefs_bufmap_desc))``
    bytes of kcalloced memory. This memory is further intialized:

      user_desc is the kernel's copy of the IO buffer's ORANGEFS_dev_map_desc
      structure. user_desc->ptr points to the IO buffer.

      ::

        pages_per_desc = bufmap->desc_size / PAGE_SIZE
        offset = 0

        bufmap->desc_array[0].page_array = &bufmap->page_array[offset]
        bufmap->desc_array[0].array_count = pages_per_desc = 1024
        bufmap->desc_array[0].uaddr = (user_desc->ptr) + (0 * 1024 * 4096)
        offset += 1024
                           .
                           .
                           .
        bufmap->desc_array[9].page_array = &bufmap->page_array[offset]
        bufmap->desc_array[9].array_count = pages_per_desc = 1024
        bufmap->desc_array[9].uaddr = (user_desc->ptr) +
                                               (9 * 1024 * 4096)
        offset += 1024

  * buffer_index_array - a desc_count sized array of ints, used to
    indicate which of the IO buffer's partitions are available to use.
  * buffer_index_lock - a spinlock to protect buffer_index_array during update.
  * readdir_index_array - a five (ORANGEFS_READDIR_DEFAULT_DESC_COUNT) element
    int array used to indicate which of the readdir buffer's partitions are
    available to use.
  * readdir_index_lock - a spinlock to protect readdir_index_array during
    update.

Operations
----------

The kernel module builds an "op" (struct orangefs_kernel_op_s) when it
needs to communicate with userspace. Part of the op contains the "upcall"
which expresses the request to userspace. Part of the op eventually
contains the "downcall" which expresses the results of the request.

The slab allocator is used to keep a cache of op structures handy.

At init time the kernel module defines and initializes a request list
and an in_progress hash table to keep track of all the ops that are
in flight at any given time.

Ops are stateful:

 * unknown
            - op was just initialized
 * waiting
            - op is on request_list (upward bound)
 * inprogr
            - op is in progress (waiting for downcall)
 * serviced
            - op has matching downcall; ok
 * purged
            - op has to start a timer since client-core
              exited uncleanly before servicing op
 * given up
            - submitter has given up waiting for it

When some arbitrary userspace program needs to perform a
filesystem operation on Orangefs (readdir, I/O, create, whatever)
an op structure is initialized and tagged with a distinguishing ID
number. The upcall part of the op is filled out, and the op is
passed to the "service_operation" function.

Service_operation changes the op's state to "waiting", puts
it on the request list, and signals the Orangefs file_operations.poll
function through a wait queue. Userspace is polling the pseudo-device
and thus becomes aware of the upcall request that needs to be read.

When the Orangefs file_operations.read function is triggered, the
request list is searched for an op that seems ready-to-process.
The op is removed from the request list. The tag from the op and
the filled-out upcall struct are copy_to_user'ed back to userspace.

If any of these (and some additional protocol) copy_to_users fail,
the op's state is set to "waiting" and the op is added back to
the request list. Otherwise, the op's state is changed to "in progress",
and the op is hashed on its tag and put onto the end of a list in the
in_progress hash table at the index the tag hashed to.

When userspace has assembled the response to the upcall, it
writes the response, which includes the distinguishing tag, back to
the pseudo device in a series of io_vecs. This triggers the Orangefs
file_operations.write_iter function to find the op with the associated
tag and remove it from the in_progress hash table. As long as the op's
state is not "canceled" or "given up", its state is set to "serviced".
The file_operations.write_iter function returns to the waiting vfs,
and back to service_operation through wait_for_matching_downcall.

Service operation returns to its caller with the op's downcall
part (the response to the upcall) filled out.

The "client-core" is the bridge between the kernel module and
userspace. The client-core is a daemon. The client-core has an
associated watchdog daemon. If the client-core is ever signaled
to die, the watchdog daemon restarts the client-core. Even though
the client-core is restarted "right away", there is a period of
time during such an event that the client-core is dead. A dead client-core
can't be triggered by the Orangefs file_operations.poll function.
Ops that pass through service_operation during a "dead spell" can timeout
on the wait queue and one attempt is made to recycle them. Obviously,
if the client-core stays dead too long, the arbitrary userspace processes
trying to use Orangefs will be negatively affected. Waiting ops
that can't be serviced will be removed from the request list and
have their states set to "given up". In-progress ops that can't
be serviced will be removed from the in_progress hash table and
have their states set to "given up".

Readdir and I/O ops are atypical with respect to their payloads.

  - readdir ops use the smaller of the two pre-allocated pre-partitioned
    memory buffers. The readdir buffer is only available to userspace.
    The kernel module obtains an index to a free partition before launching
    a readdir op. Userspace deposits the results into the indexed partition
    and then writes them to back to the pvfs device.

  - io (read and write) ops use the larger of the two pre-allocated
    pre-partitioned memory buffers. The IO buffer is accessible from
    both userspace and the kernel module. The kernel module obtains an
    index to a free partition before launching an io op. The kernel module
    deposits write data into the indexed partition, to be consumed
    directly by userspace. Userspace deposits the results of read
    requests into the indexed partition, to be consumed directly
    by the kernel module.

Responses to kernel requests are all packaged in pvfs2_downcall_t
structs. Besides a few other members, pvfs2_downcall_t contains a
union of structs, each of which is associated with a particular
response type.

The several members outside of the union are:

 ``int32_t type``
    - type of operation.
 ``int32_t status``
    - return code for the operation.
 ``int64_t trailer_size``
    - 0 unless readdir operation.
 ``char *trailer_buf``
    - initialized to NULL, used during readdir operations.

The appropriate member inside the union is filled out for any
particular response.

  PVFS2_VFS_OP_FILE_IO
    fill a pvfs2_io_response_t

  PVFS2_VFS_OP_LOOKUP
    fill a PVFS_object_kref

  PVFS2_VFS_OP_CREATE
    fill a PVFS_object_kref

  PVFS2_VFS_OP_SYMLINK
    fill a PVFS_object_kref

  PVFS2_VFS_OP_GETATTR
    fill in a PVFS_sys_attr_s (tons of stuff the kernel doesn't need)
    fill in a string with the link target when the object is a symlink.

  PVFS2_VFS_OP_MKDIR
    fill a PVFS_object_kref

  PVFS2_VFS_OP_STATFS
    fill a pvfs2_statfs_response_t with useless info <g>. It is hard for
    us to know, in a timely fashion, these statistics about our
    distributed network filesystem.

  PVFS2_VFS_OP_FS_MOUNT
    fill a pvfs2_fs_mount_response_t which is just like a PVFS_object_kref
    except its members are in a different order and "__pad1" is replaced
    with "id".

  PVFS2_VFS_OP_GETXATTR
    fill a pvfs2_getxattr_response_t

  PVFS2_VFS_OP_LISTXATTR
    fill a pvfs2_listxattr_response_t

  PVFS2_VFS_OP_PARAM
    fill a pvfs2_param_response_t

  PVFS2_VFS_OP_PERF_COUNT
    fill a pvfs2_perf_count_response_t

  PVFS2_VFS_OP_FSKEY
    file a pvfs2_fs_key_response_t

  PVFS2_VFS_OP_READDIR
    jamb everything needed to represent a pvfs2_readdir_response_t into
    the readdir buffer descriptor specified in the upcall.

Userspace uses writev() on /dev/pvfs2-req to pass responses to the requests
made by the kernel side.

A buffer_list containing:

  - a pointer to the prepared response to the request from the
    kernel (struct pvfs2_downcall_t).
  - and also, in the case of a readdir request, a pointer to a
    buffer containing descriptors for the objects in the target
    directory.

... is sent to the function (PINT_dev_write_list) which performs
the writev.

PINT_dev_write_list has a local iovec array: struct iovec io_array[10];

The first four elements of io_array are initialized like this for all
responses::

  io_array[0].iov_base = address of local variable "proto_ver" (int32_t)
  io_array[0].iov_len = sizeof(int32_t)

  io_array[1].iov_base = address of global variable "pdev_magic" (int32_t)
  io_array[1].iov_len = sizeof(int32_t)

  io_array[2].iov_base = address of parameter "tag" (PVFS_id_gen_t)
  io_array[2].iov_len = sizeof(int64_t)

  io_array[3].iov_base = address of out_downcall member (pvfs2_downcall_t)
                         of global variable vfs_request (vfs_request_t)
  io_array[3].iov_len = sizeof(pvfs2_downcall_t)

Readdir responses initialize the fifth element io_array like this::

  io_array[4].iov_base = contents of member trailer_buf (char *)
                         from out_downcall member of global variable
                         vfs_request
  io_array[4].iov_len = contents of member trailer_size (PVFS_size)
                        from out_downcall member of global variable
                        vfs_request

Orangefs exploits the dcache in order to avoid sending redundant
requests to userspace. We keep object inode attributes up-to-date with
orangefs_inode_getattr. Orangefs_inode_getattr uses two arguments to
help it decide whether or not to update an inode: "new" and "bypass".
Orangefs keeps private data in an object's inode that includes a short
timeout value, getattr_time, which allows any iteration of
orangefs_inode_getattr to know how long it has been since the inode was
updated. When the object is not new (new == 0) and the bypass flag is not
set (bypass == 0) orangefs_inode_getattr returns without updating the inode
if getattr_time has not timed out. Getattr_time is updated each time the
inode is updated.

Creation of a new object (file, dir, sym-link) includes the evaluation of
its pathname, resulting in a negative directory entry for the object.
A new inode is allocated and associated with the dentry, turning it from
a negative dentry into a "productive full member of society". Orangefs
obtains the new inode from Linux with new_inode() and associates
the inode with the dentry by sending the pair back to Linux with
d_instantiate().

The evaluation of a pathname for an object resolves to its corresponding
dentry. If there is no corresponding dentry, one is created for it in
the dcache. Whenever a dentry is modified or verified Orangefs stores a
short timeout value in the dentry's d_time, and the dentry will be trusted
for that amount of time. Orangefs is a network filesystem, and objects
can potentially change out-of-band with any particular Orangefs kernel module
instance, so trusting a dentry is risky. The alternative to trusting
dentries is to always obtain the needed information from userspace - at
least a trip to the client-core, maybe to the servers. Obtaining information
from a dentry is cheap, obtaining it from userspace is relatively expensive,
hence the motivation to use the dentry when possible.

The timeout values d_time and getattr_time are jiffy based, and the
code is designed to avoid the jiffy-wrap problem::

    "In general, if the clock may have wrapped around more than once, there
    is no way to tell how much time has elapsed. However, if the times t1
    and t2 are known to be fairly close, we can reliably compute the
    difference in a way that takes into account the possibility that the
    clock may have wrapped between times."

from course notes by instructor Andy Wang