On Tue, Mar 05, 2024 at 02:43:32PM -0300, Fabiano Rosas wrote:
> Commit bc38feddeb ("io: fsync before closing a file channel") added a
> fsync/fdatasync at the closing point of the QIOChannelFile to ensure
> integrity of the migration stream in case of QEMU crash.
>
> The decision to do the sync at qio_channel_close() was not the best
> since that function runs in the main thread and the fsync can cause
> QEMU to hang for several minutes, depending on the migration size and
> disk speed.
>
> To fix the hang, remove the fsync from qio_channel_file_close().
>
> At this moment, the migration code is the only user of the fsync and
> we're taking the tradeoff of not having a sync at all, leaving the
> responsibility to the upper layers.
>
> Fixes: bc38feddeb ("io: fsync before closing a file channel")
> Signed-off-by: Fabiano Rosas <faro...@suse.de>
> ---
> docs/devel/migration/main.rst | 3 ++-
> io/channel-file.c | 5 -----
> migration/multifd.c | 13 -------------
> 3 files changed, 2 insertions(+), 19 deletions(-)
>
> diff --git a/docs/devel/migration/main.rst b/docs/devel/migration/main.rst
> index 8024275d6d..54385a23e5 100644
> --- a/docs/devel/migration/main.rst
> +++ b/docs/devel/migration/main.rst
> @@ -44,7 +44,8 @@ over any transport.
> - file migration: do the migration using a file that is passed to QEMU
> by path. A file offset option is supported to allow a management
> application to add its own metadata to the start of the file without
> - QEMU interference.
> + QEMU interference. Note that QEMU does not flush cached file
> + data/metadata at the end of migration.
>
> In addition, support is included for migration using RDMA, which
> transports the page data using ``RDMA``, where the hardware takes care of
> diff --git a/io/channel-file.c b/io/channel-file.c
> index d4706fa592..a6ad7770c6 100644
> --- a/io/channel-file.c
> +++ b/io/channel-file.c
> @@ -242,11 +242,6 @@ static int qio_channel_file_close(QIOChannel *ioc,
> {
> QIOChannelFile *fioc = QIO_CHANNEL_FILE(ioc);
>
> - if (qemu_fdatasync(fioc->fd) < 0) {
> - error_setg_errno(errp, errno,
> - "Unable to synchronize file data with storage
> device");
> - return -1;
> - }
> if (qemu_close(fioc->fd) < 0) {
> error_setg_errno(errp, errno,
> "Unable to close file");
Upto here:
Reviewed-by: Daniel P. Berrangé <berra...@redhat.com>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index d4a44da559..2edcd5104e 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -709,19 +709,6 @@ static bool
> multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
> {
> if (p->c) {
> migration_ioc_unregister_yank(p->c);
> - /*
> - * An explicit close() on the channel here is normally not
> - * required, but can be helpful for "file:" iochannels, where it
> - * will include fdatasync() to make sure the data is flushed to the
> - * disk backend.
> - *
> - * The object_unref() cannot guarantee that because: (1) finalize()
> - * of the iochannel is only triggered on the last reference, and
> - * it's not guaranteed that we always hold the last refcount when
> - * reaching here, and, (2) even if finalize() is invoked, it only
> - * does a close(fd) without data flush.
> - */
> - qio_channel_close(p->c, &error_abort);
> object_unref(OBJECT(p->c));
> p->c = NULL;
> }
I don't think you should be removing this. Calling qio_channel_close()
remains recommended best practice, even with fdatasync() removed, as
it provides a strong guarantee that the FD is released which you don't
get if you rely on the ref count being correctly decremented in all
code paths.
With regards,
Daniel
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