Adding O_DIRECT support to ZFS to bypass the ARC for writes/reads.
O_DIRECT support in ZFS will always ensure there is coherency between
buffered and O_DIRECT IO requests. This ensures that all IO requests,
whether buffered or direct, will see the same file contents at all
times. Just as in other FS's , O_DIRECT does not imply O_SYNC. While
data is written directly to VDEV disks, metadata will not be synced
until the associated TXG is synced.
For both O_DIRECT read and write request the offset and request sizes,
at a minimum, must be PAGE_SIZE aligned. In the event they are not,
then EINVAL is returned unless the direct property is set to always (see
below).
For O_DIRECT writes:
The request also must be block aligned (recordsize) or the write
request will take the normal (buffered) write path. In the event that
request is block aligned and a cached copy of the buffer in the ARC,
then it will be discarded from the ARC forcing all further reads to
retrieve the data from disk.
For O_DIRECT reads:
The only alignment restrictions are PAGE_SIZE alignment. In the event
that the requested data is in buffered (in the ARC) it will just be
copied from the ARC into the user buffer.
For both O_DIRECT writes and reads the O_DIRECT flag will be ignored in
the event that file contents are mmap'ed. In this case, all requests
that are at least PAGE_SIZE aligned will just fall back to the buffered
paths. If the request however is not PAGE_SIZE aligned, EINVAL will
be returned as always regardless if the file's contents are mmap'ed.
Since O_DIRECT writes go through the normal ZIO pipeline, the
following operations are supported just as with normal buffered writes:
Checksum
Compression
Encryption
Erasure Coding
There is one caveat for the data integrity of O_DIRECT writes that is
distinct for each of the OS's supported by ZFS.
FreeBSD - FreeBSD is able to place user pages under write protection so
any data in the user buffers and written directly down to the
VDEV disks is guaranteed to not change. There is no concern
with data integrity and O_DIRECT writes.
Linux - Linux is not able to place anonymous user pages under write
protection. Because of this, if the user decides to manipulate
the page contents while the write operation is occurring, data
integrity can not be guaranteed. However, there is a module
parameter `zfs_vdev_direct_write_verify` that controls the
if a O_DIRECT writes that can occur to a top-level VDEV before
a checksum verify is run before the contents of the I/O buffer
are committed to disk. In the event of a checksum verification
failure the write will return EIO. The number of O_DIRECT write
checksum verification errors can be observed by doing
`zpool status -d`, which will list all verification errors that
have occurred on a top-level VDEV. Along with `zpool status`, a
ZED event will be issues as `dio_verify` when a checksum
verification error occurs.
ZVOLs and dedup is not currently supported with Direct I/O.
A new dataset property `direct` has been added with the following 3
allowable values:
disabled - Accepts O_DIRECT flag, but silently ignores it and treats
the request as a buffered IO request.
standard - Follows the alignment restrictions outlined above for
write/read IO requests when the O_DIRECT flag is used.
always - Treats every write/read IO request as though it passed
O_DIRECT and will do O_DIRECT if the alignment restrictions
are met otherwise will redirect through the ARC. This
property will not allow a request to fail.
There is also a module parameter zfs_dio_enabled that can be used to
force all reads and writes through the ARC. By setting this module
parameter to 0, it mimics as if the direct dataset property is set to
disabled.
Reviewed-by: Brian Behlendorf <behlendorf@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Co-authored-by: Mark Maybee <mark.maybee@delphix.com>
Co-authored-by: Matt Macy <mmacy@FreeBSD.org>
Co-authored-by: Brian Behlendorf <behlendorf@llnl.gov>
Closes#10018
The kernel is now being compiled with -Wmissing-prototypes. Most of our
test stub functions had no prototype, and failed to compile. Since they
don't need to be visible anywhere else, just make them all static.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/Closes#15805
Reduce the time required for ./configure to perform the needed
KABI checks by allowing kbuild to compile multiple test cases in
parallel. This was accomplished by splitting each test's source
code from the logic handling whether that code could be compiled
or not.
By introducing this split it's possible to minimize the number of
times kbuild needs to be invoked. As importantly, it means all of
the tests can be built in parallel. This does require a little extra
care since we expect some tests to fail, so the --keep-going (-k)
option must be provided otherwise some tests may not get compiled.
Furthermore, since a failure during the kbuild modpost phase will
result in an early exit; the final linking phase is limited to tests
which passed the initial compilation and produced an object file.
Once everything has been built the configure script proceeds as
previously. The only significant difference is that it now merely
needs to test for the existence of a .ko file to determine the
result of a given test. This vastly speeds up the entire process.
New test cases should use ZFS_LINUX_TEST_SRC to declare their test
source code and ZFS_LINUX_TEST_RESULT to check the result. All of
the existing kernel-*.m4 files have been updated accordingly, see
config/kernel-current-time.m4 for a basic example. The legacy
ZFS_LINUX_TRY_COMPILE macro has been kept to handle special cases
but it's use is not encouraged.
master (secs) patched (secs)
------------- ----------------
autogen.sh 61 68
configure 137 24 (~17% of current run time)
make -j $(nproc) 44 44
make rpms 287 150
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#8547Closes#9132Closes#9341
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#224Closes#7823
