Kernel Asynchronous I/O (AIO) Support for Linux
Overview
AIO enables even a single application thread to overlap I/O
operations with other processing, by providing an interface for
submitting one or more I/O requests in one system call (io_submit())
without waiting for completion, and a separate interface (io_getevents())
to reap completed I/O operations associated with a given completion
group.
Support for kernel AIO has been included in the 2.6 Linux kernel.
What Works?
- AIO read and write on raw (and O_DIRECT on blockdev)
- AIO read and write on files opened with O_DIRECT on ext2, ext3,
jfs, xfs
What Does Not Work?
- AIO read and write on files opened without O_DIRECT (i.e. normal
buffered filesystem AIO). On ext2, ext3, jfs, xfs and nfs, these
do not return an explicit error, but quietly default to
synchronous or rather non-AIO behaviour (i.e io_submit waits for I/O to
complete in these cases). For most other filesystems, -EINVAL is
reported.
- AIO fsync (not supported for any filesystem)
- AIO read and write on sockets (doesn't return an explicit error,
but quietly defaults to synchronous or rather non-AIO behavior)
- AIO read and write on pipes (reports -EINVAL)
- Not all devices (including TTY) support AIO (typically return
-EINVAL)
Additional functionality is available HERE
- Buffered filesystem AIO, i.e. AIO read and write on files opened
without O_DIRECT, on ext2, ext3, jfs, xfs, reiserfs.
- Does not include AIO fsync
- AIO read and write on pipes (from Chris Mason)
Q. Where can I find AIO libraries that utilize kernel AIO on
Linux ?
A. You need to install the libaio package if you are writing AIO
applications which use the native AIO interfaces (libaio-0.3.99)
Q. Is AIO functionality available for 2.4 kernels?
A. Patches for 2.4 can be found
HERE . Also check out the original Linux-AIO home page.
Reading About the Linux Kernel AIO Design
-
Asynchronous I/O Support Linux 2.5, Ottawa Linux Symposium 2003
Covers the AIO design for 2.6, including the filesystem AIO
implementation that's currently in the -mm tree, and some performance
results for O_DIRECT AIO using rawiobench.
- AIO Design
Notes written at the time of the 2.4 AIO implementation, These are a
little out-of-date, but provide some idea and background on various
issues, alternative design approaches and considerations involved in
an AIO implementation and describes the design of the 2.4 AIO
implementation.
- Round
3: aio vs /dev/epoll compares epoll, AIO poll and AIO read
behaviour. The full paper is included in the OLS
2002 Proceedings.
Future Work
- Performance Tuning
- Vector AIO
Benchmarks and Testing
Performance testing/microbenchmarking
- Chris Mason's
aio-stress benchmark issues a stream of AIO requests to one or more
files, where one can vary several parameters like I/O unit size, total
I/O size, depth of iocbs submitted at a time, number of concurrent
threads, type and pattern of I/O operations, etc. It reports the
overall throughput attained and average io submission latency.
-
rawiobench is another microbenchmark which can issue sequential or
random disk I/O. It supports both synchronous and asynchronous I/O for
O_DIRECT and buffered filesystem AIO and reports throughput and CPU
utilization results.
Other tests AIO tests:
- Daniel McNiel's (OSDL) aiocp test.
- fsx-linux
is now enabled for AIO. Just compile it with -DAIO.
Performance Results
- Filesystem AIO aiostress results
with and without fsaio patches
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