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 <stefa...@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 <h...@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
