Hello Stefan,
Can you expand on this some more? I have similar concerns for Livebackup.
At the beginning of your paragraph, did you mean 'asynchronous I/O
emulation' instead of 'synchronous I/O emulation'?
Also, I don't understand the 'stack' construct that you refer to. When you
say 'push a new context', are you talking about what happens when a new
thread picks up a new async I/O req from the VM, and then proceeds to
execute the I/O req? What is this stack that you refer to?
Any design documents, code snippets that I can look, other pointers welcome.
See async.c.
Thanks - I will do so.
There is synchronous I/O emulation in block.c for BlockDrivers that
don't support .bdrv_read()/.bdrv_write() but only
.bdrv_aio_readv()/.bdrv_aio_writev(). The way it works is that it
pushes a new I/O context and then issues async I/O. Then it runs a
special event loop waiting for that I/O to complete. After the I/O
completes it pops the context again.
OK. This is the opposite of what I was thinking of. I was considering
the code that emulates Async I/O using multiple threads.
It sounds like the goal of this async.c mechanism is more than
serializing all synchronous I/O requests, right?
The point of the context is that completions only get called for the
current context. Therefore callers of the synchronous I/O functions
don't need to worry that the state of the world might change during
their "synchronous" operation - only their own I/O operation can
complete. If a pending async I/O completes during synchronous I/O
emulation, its callback is not invoked until the bottom half (BH) is
called after the async context is popped. This guarantees that the
synchronous operation and its caller have completed before I/O
completion callbacks are invoked for pending async I/O.
OK. This might not be a problem for Livebackup, after all.
Livebackup transparently interposes the async I/O offered by the driver.
Hence the async.c mechanism is layered neatly above the
async_block_driver+livebackup layer and should work correctly.
I will take a closer look at this, and check for sanity...
Thanks,
Jagane
