[Stgt-devel] [PATCH] Make changes so that VTL stuff works properly ...

[ronnie sahlberg]

On Fri, Aug 8, 2008 at 4:14 AM, FUJITA Tomonori
<fujita.tomonori at lab.ntt.co.jp> wrote:
> On Wed, 6 Aug 2008 18:06:26 +0300
> "Alexander Nezhinsky" <nezhinsky at gmail.com> wrote:
>
>> On Wed, Jul 30, 2008 at 10:04 AM, FUJITA Tomonori
>> <fujita.tomonori at lab.ntt.co.jp> wrote:
>>
>> > the future... We have it just because Linux AIO is not good enough
>> > now.
>>
>> Can you explain a bit what's wrong with the current Linux AIO implementation.
>> Why bs_aio.c is implemented using syscall(__NR_io_*)?
>> What is wrong with the aio_read/aio_write and friends?
>
>
> I think that that's Posix AIO, which glibc implemented. It's slower
> than the kernel AIO support. A big problem about it for us is event
> notification. We can't get AIO completion events via a file descriptor
> (so that we can't use poll/epoll with it). How can we wait for both
> network events and AIO events at the same time?

What about setting up a pipe, forking, doing the the i/o in the child
and then writing a byte of data to the pipe when the command completed.
The parent can then "wait for the 'AIO' to complete by waiting on a
file descriptor" like normal.

forks are cheap under linux.

I do this all the time in ctdb which is very hard real time and which
can not ever call any function that may block from the main daemon.
(== read/write/fcntl/waitpid,...)


This is how samba 3 does AIO as well   since the real AIO layer on
many distribution versions
are "suboptimal".