OpenZFS on Linux and FreeBSD
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Brian Behlendorf a584ef2605
Direct IO support
Direct IO via the O_DIRECT flag was originally introduced in XFS by
IRIX for database workloads. Its purpose was to allow the database
to bypass the page and buffer caches to prevent unnecessary IO
operations (e.g.  readahead) while preventing contention for system
memory between the database and kernel caches.

On Illumos, there is a library function called directio(3C) that
allows user space to provide a hint to the file system that Direct IO
is useful, but the file system is free to ignore it. The semantics
are also entirely a file system decision. Those that do not
implement it return ENOTTY.

Since the semantics were never defined in any standard, O_DIRECT is
implemented such that it conforms to the behavior described in the
Linux open(2) man page as follows.

    1.  Minimize cache effects of the I/O.

    By design the ARC is already scan-resistant which helps mitigate
    the need for special O_DIRECT handling.  Data which is only
    accessed once will be the first to be evicted from the cache.
    This behavior is in consistent with Illumos and FreeBSD.

    Future performance work may wish to investigate the benefits of
    immediately evicting data from the cache which has been read or
    written with the O_DIRECT flag.  Functionally this behavior is
    very similar to applying the 'primarycache=metadata' property
    per open file.

    2. O_DIRECT _MAY_ impose restrictions on IO alignment and length.

    No additional alignment or length restrictions are imposed.

    3. O_DIRECT _MAY_ perform unbuffered IO operations directly
       between user memory and block device.

    No unbuffered IO operations are currently supported.  In order
    to support features such as transparent compression, encryption,
    and checksumming a copy must be made to transform the data.

    4. O_DIRECT _MAY_ imply O_DSYNC (XFS).

    O_DIRECT does not imply O_DSYNC for ZFS.  Callers must provide
    O_DSYNC to request synchronous semantics.

    5. O_DIRECT _MAY_ disable file locking that serializes IO
       operations.  Applications should avoid mixing O_DIRECT
       and normal IO or mmap(2) IO to the same file.  This is
       particularly true for overlapping regions.

    All I/O in ZFS is locked for correctness and this locking is not
    disabled by O_DIRECT.  However, concurrently mixing O_DIRECT,
    mmap(2), and normal I/O on the same file is not recommended.

This change is implemented by layering the aops->direct_IO operations
on the existing AIO operations.  Code already existed in ZFS on Linux
for bypassing the page cache when O_DIRECT is specified.

References:
  * http://xfs.org/docs/xfsdocs-xml-dev/XFS_User_Guide/tmp/en-US/html/ch02s09.html
  * https://blogs.oracle.com/roch/entry/zfs_and_directio
  * https://ext4.wiki.kernel.org/index.php/Clarifying_Direct_IO's_Semantics
  * https://illumos.org/man/3c/directio

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #224 
Closes #7823
2018-08-27 10:04:21 -07:00
.github PR's should provide motivation & context first 2018-07-23 22:08:18 -07:00
cmd Added metadata/dnode cache info to arc_summary 2018-08-22 09:35:20 -07:00
config Direct IO support 2018-08-27 10:04:21 -07:00
contrib pyzfs: add missing libzfs_core functions 2018-08-20 10:11:52 -07:00
etc Minor documentation, logging, and testing typos 2018-06-07 09:38:39 -07:00
include Introduce read/write kstats per dataset 2018-08-20 09:52:37 -07:00
lib Fedora 28: Fix misc bounds check compiler warnings 2018-08-26 12:55:44 -07:00
man Introduce read/write kstats per dataset 2018-08-20 09:52:37 -07:00
module Direct IO support 2018-08-27 10:04:21 -07:00
rpm Remove %changelog from spec file 2018-08-26 12:59:33 -07:00
scripts Direct IO support 2018-08-27 10:04:21 -07:00
tests Direct IO support 2018-08-27 10:04:21 -07:00
udev Add kernel module auto-loading 2018-03-13 10:45:55 -07:00
.gitignore Ignore *.o.ur-safe build artifacts 2018-05-13 18:59:02 -07:00
.gitmodules Add zimport.sh compatibility test script 2014-02-21 12:10:31 -08:00
.travis.yml Add .travis.yml 2017-11-13 09:18:18 -08:00
AUTHORS Update build system and packaging 2018-05-29 16:00:33 -07:00
COPYRIGHT Update build system and packaging 2018-05-29 16:00:33 -07:00
LICENSE Update build system and packaging 2018-05-29 16:00:33 -07:00
META Linux compat 4.18: check_disk_size_change() 2018-06-15 15:05:21 -07:00
Makefile.am make install only works once 2018-05-31 09:19:59 -07:00
NOTICE Update build system and packaging 2018-05-29 16:00:33 -07:00
README.md Explicitly state supported Linux versions 2018-05-30 20:11:19 -07:00
TEST Update build system and packaging 2018-05-29 16:00:33 -07:00
autogen.sh Cause autogen.sh to fail if autoreconf fails 2018-07-06 09:27:37 -07:00
configure.ac Direct IO support 2018-08-27 10:04:21 -07:00
copy-builtin copy-builtin: SPL must be in Kbuild first 2018-06-15 15:16:29 -07:00
zfs.release.in Move zfs.release generation to configure step 2012-07-12 12:22:51 -07:00

README.md

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ZFS on Linux is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community.

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Official Resources

Installation

Full documentation for installing ZoL on your favorite Linux distribution can be found at our site.

Contribute & Develop

We have a separate document with contribution guidelines.

Release

ZFS on Linux is released under a CDDL license.
For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197

Supported Kernels

  • The META file contains the officially recognized supported kernel versions.