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