mirror of
https://github.com/ventoy/Ventoy.git
synced 2024-11-06 03:20:39 +00:00
32 lines
1.9 KiB
Plaintext
32 lines
1.9 KiB
Plaintext
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 <chris@arachsys.com>, 2008-04-21.
|