Re: Qemu first time contribution

[Harshavardhan Unnibhavi]

Hi Stefan,

This looks interesting, let me take a look at it. Thank you!

Best,
Harsha

On Mon, Nov 9, 2020 at 10:17 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Sun, Nov 08, 2020 at 12:21:33PM +0000, Harshavardhan Unnibhavi wrote:
> > Thank you for the reply! Yes, I understand that gsoc is over for 2020,
> > and projects for 2021 will come up next year. I was thinking of
> > contributing outside of gsoc(for which I won't be eligible anyways for
> > next year). Anyway, I will work on some of the bite sized tasks, and
> > get back to you for some other concrete project ideas that require
> > somebody to work on, in qemu.
>
> Hi Harsha,
> Here is an idea you could explore:
>
> The Linux AIO API was extended to support fsync(2)/fdatasync(2) in the
> following commit from 2018:
>
>   commit a3c0d439e4d92411c2b4b21a526a4de720d0806b
>   Author: Christoph Hellwig <hch@lst.de>
>   Date:   Tue Mar 27 19:18:57 2018 +0200
>
>       aio: implement IOCB_CMD_FSYNC and IOCB_CMD_FDSYNC
>
> QEMU's Linux AIO code does not take advantage of this feature yet.
> Instead it invokes the traditional fdatasync(2) system call from a
> thread pool because it assumes the Linux AIO API doesn't support the
> operation. The function where this happens is
> block/file-posix.c:raw_co_flush_to_disk().
>
> The goal is to implement IO_CMD_FDSYNC support in block/linux-aio.c
> using io_prep_fdsync() and update
> block/file-posix.c:raw_co_flush_to_disk() to use this when the feature
> is available. See <libaio.h> for the Linux AIO library API.
>
> Keep in mind that old host kernels may not support IO_CMD_FDSYNC. In
> that case QEMU should continue to use the thread pool.
>
> Taking advantage of the Linux AIO API means QEMU will spawn fewer
> worker threads and disk flush performance may improve. You can benchmark
> performance using the fio(1) tool. Configure it with ioengine=pvsync2
> rw=randwrite direct=1 fdatasync=1 bs=4k to measure the peformance of 4
> KB writes followed by fdatasync. For more information about disk I/O
> benchmarking, including example fio jobs, see:
> https://blog.vmsplice.net/2017/11/common-disk-benchmarking-mistakes.html
>
> Stefan