aio_read、aio_write 是否被内核缓冲？在 Linux 的情况下，它们是否通过页面缓存？
Are aio_read, aio_write buffered by kernel? In case of Linux, does they go through page cache?
问题如题。
在 Linux 中，传递 O_DIRECT 绕过 buffer 完成的页面缓存。比如数据库系统会用它来管理自己的buffer。
我想知道 aio_write/aio_read 是否默认缓冲？他们当然要排队，但这是否涉及复制数据？如果它们默认被缓冲，可能我们可以只 open 文件而不传递 O_DIRECT，尽管在任何情况下传递 O_DIRECT 都会绕过普通文件的内核页面缓存。
POSIX standards 似乎暗示 aio_read/aio_write 没有被内核缓冲：
Effect of Buffering
If asynchronous I/O is performed on a file that is buffered prior to
being actually written to the device, it is possible that asynchronous
I/O will offer no performance advantage over normal I/O; the cycles
stolen to perform the asynchronous I/O will be taken away from the
running process and the I/O will occur at interrupt time. This
potential lack of gain in performance in no way obviates the need for
asynchronous I/O by realtime applications, which very often will use
specialized hardware support, multiple processors, and/or unbuffered,
synchronized I/O.
但是，在描述aio_read/aio_write关于缓冲的时候，没有明确的说法。
是的。他们通过内核缓存。 aio_fsync 证明了这一点。
问题如题。
在 Linux 中，传递 O_DIRECT 绕过 buffer 完成的页面缓存。比如数据库系统会用它来管理自己的buffer。
我想知道 aio_write/aio_read 是否默认缓冲？他们当然要排队，但这是否涉及复制数据？如果它们默认被缓冲，可能我们可以只 open 文件而不传递 O_DIRECT，尽管在任何情况下传递 O_DIRECT 都会绕过普通文件的内核页面缓存。
POSIX standards 似乎暗示 aio_read/aio_write 没有被内核缓冲：
Effect of Buffering
If asynchronous I/O is performed on a file that is buffered prior to being actually written to the device, it is possible that asynchronous I/O will offer no performance advantage over normal I/O; the cycles stolen to perform the asynchronous I/O will be taken away from the running process and the I/O will occur at interrupt time. This potential lack of gain in performance in no way obviates the need for asynchronous I/O by realtime applications, which very often will use specialized hardware support, multiple processors, and/or unbuffered, synchronized I/O.
但是，在描述aio_read/aio_write关于缓冲的时候，没有明确的说法。
是的。他们通过内核缓存。 aio_fsync 证明了这一点。