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32 lines
1.9 KiB
Plaintext
32 lines
1.9 KiB
Plaintext
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This proof-of-concept patch modifies vblade to access the underlying block
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device using POSIX asynchronous IO (AIO) rather than using normal blocking
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read() and write(). AIO allows vblade to receive and queue several several ATA
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read/write commands at once, returning the response to the client
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asynchronously as each IO operation completes. It should be most beneficial
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for devices which experience very non-sequential IO. An AIO-enabled vblade is
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also a good starting point if you want to generalise vblade to export multiple
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devices without the complexity and overhead of a multithreaded approach.
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The patch implements AIO support for both Linux and FreeBSD, but I have not
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tested the FreeBSD support and would therefore be especially interested to
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hear success/failure reports for compiling and running AIO vblade on FreeBSD.
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A SIGIO handler which writes a single byte to a pipe is used to notify the
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main poll() loop that AIO operations have completed and are ready to return to
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the client. Running oprofile on a box with a heavily loaded loopback
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vblade-aio suggests that it spends an inordinate amount of time in the signal
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handler. Some method of poll()ing directly on the AIO events at the same time
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as the socket fd could cut this overhead out completely.
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More generally, experimenting on Linux with standard O_DIRECT vblade and
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O_DIRECT vblade-aio on a loopback interface with MTU 9000 suggests that the
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performance difference on a single RAID1-backed block device is fairly small:
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swamped by the performance of the network and the underlying block device.
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However, the POSIX AIO in glibc librt is emulated in userspace threads rather
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than using the kernel AIO api. A kernel-backed POSIX AIO implementation should
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perform better, especially for multiple access to a single block device.
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I would be delighted to hear any feedback and experiences from people running
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vblade together with this patch.
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Chris Webb <chris@arachsys.com>, 2008-04-21.
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