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Discussion: Asynchronous IO
for Linux in 2.5

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Benjamin LaHaise
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bcrl@redhat.com

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Discussion Goals
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	bits & state of merge
	implementation experiences
	api fun
	how do we proceed?

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Bits
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	wait queue function callbacks (merged)
	new syscalls (fs/aio.c -- not yet merged)
	work to dos (not merged, needs to switch to tasklets?)
	kvec (not merged, questionable direction?)
	generic file read / write operations (not yet merged, rewrite?)
	non-fast path: drivers (not done yet, how?)

%%		io_setup(nr_reqs, *ctx)
%%		io_destroy(ctx)
%%		io_submit(ctx, nr, **iocbs)
%%		io_cancel2(ctx, *iocb, *event)
%%		io_getevents(ctx, nr, *events, *timeout)
%%
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Implementation Experiences
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	syscalls worked well
	tqueues for worktodos were a Bad Idea (tm)
	refcounting iocbs is not optional
	code duplication in aio_read/write operations
	kvecs / kiobufs suck for small and large ios
	fast paths are important
	new_read/write doesn't work
	explicitly async operations can be large

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API fun
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	file_operations
		ssize_t aio_read(struct kiocb *, char *, size_t, loff_t);
		ssize_t aio_write(struct kiocb *, const char *, size_t, loff_t);
		int aio_fsync(struct kiocb *, int datasync);

	address_space_operations
		read/write/sync_ page
		read/write pages
		direct_IO
		prepare/commit_write
		set_page_dirty

	mmm, code duplication

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How do we proceed?
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	a true commitment to aio support will impact the entire kernel
	how far should we go?
	should support for unimplemented operations be in user or kernel space?