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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
66 lines
1.8 KiB
Bash
Executable File
66 lines
1.8 KiB
Bash
Executable File
#! /bin/ksh -p
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#
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# CDDL HEADER START
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#
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# The contents of this file are subject to the terms of the
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# Common Development and Distribution License (the "License").
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# You may not use this file except in compliance with the License.
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#
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# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
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# or http://www.opensolaris.org/os/licensing.
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# See the License for the specific language governing permissions
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# and limitations under the License.
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#
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# When distributing Covered Code, include this CDDL HEADER in each
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# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
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# If applicable, add the following below this CDDL HEADER, with the
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# fields enclosed by brackets "[]" replaced with your own identifying
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# information: Portions Copyright [yyyy] [name of copyright owner]
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#
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# CDDL HEADER END
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#
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#
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# Copyright (c) 2018 by Lawrence Livermore National Security, LLC.
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#
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. $STF_SUITE/include/libtest.shlib
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. $STF_SUITE/tests/functional/io/io.cfg
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#
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# DESCRIPTION:
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# Verify Linux native asynchronous IO.
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#
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# STRATEGY:
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# 1. Use fio(1) in verify mode to perform write, read,
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# random read, and random write workloads.
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# 2. Repeat the test with additional fio(1) options.
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#
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verify_runnable "global"
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function cleanup
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{
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log_must rm -f "$mntpnt/rw*"
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}
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log_assert "Verify Linux native asynchronous IO"
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log_onexit cleanup
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ioengine="--ioengine=libaio"
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mntpnt=$(get_prop mountpoint $TESTPOOL/$TESTFS)
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dir="--directory=$mntpnt"
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set -A fio_arg -- "--sync=0" "--sync=1" "--direct=0" "--direct=1"
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for arg in "${fio_arg[@]}"; do
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log_must fio $dir $ioengine $arg $FIO_WRITE_ARGS
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log_must fio $dir $ioengine $arg $FIO_READ_ARGS
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log_must fio $dir $ioengine $arg $FIO_RANDWRITE_ARGS
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log_must fio $dir $ioengine $arg $FIO_RANDREAD_ARGS
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log_must rm -f "$mntpnt/rw*"
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done
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log_pass "Verified Linux native asynchronous IO"
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