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Date: Mon, 03 Dec 2007 15:36:42 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH 2/2 v2] Direct IDE I/O
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To: qemu-devel@nongnu.org
Cc: Laurent Vivier <Laurent.Vivier@bull.net>

Paul Brook wrote:
>> Yes, librt is providing posix-aio, and librt coming with GNU libc uses
>> threads.
>> But if I remember correctly librt coming with RHEL uses a mix of threads
>> and linux kernel AIO (you can have a look to the .srpm of libc).
>>
>> BTW, if everyone thinks it could be a good idea I can port block-raw.c
>> to use linux kernel AIO (without removing POSIX AIO support, of course)
>>     
>
> This seems rather pointless, given a user can just use a linux-AIO librt 
> instead.
>   

Not at all.  linux-aio is the only interface that allows you to do 
asynchronous fdsync which simulates a barrier which allows for an 
ordered queue.

I have a patch that uses linux-aio for the virtio-blk driver I'll be 
posting tomorrow and I'm extremely happy with the results.  In recent 
kernels, you can use an eventfd interface along with linux-aio so that 
polling is unnecessary.  Along with O_DIRECT and the preadv/pwritev 
interface, you can make a block backend in userspace that performs just 
as well as if it were in the kernel.

The posix-aio interface simply doesn't provide a mechanism to do these 
things.

Regards,

Anthony Liguori

> Paul
>
>
>
>