Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread

[Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>]

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On 23.03.2021 17:54, Peter Xu wrote:
> On Tue, Mar 23, 2021 at 10:51:57AM +0300, Andrey Gruzdev wrote:
>> On 22.03.2021 23:17, Peter Xu wrote:
>>> On Fri, Mar 19, 2021 at 05:52:47PM +0300, Andrey Gruzdev wrote:
>>>> Added missing qemu_fflush() on buffer file holding precopy device state.
>>>> Increased initial QIOChannelBuffer allocation to 512KB to avoid reallocs.
>>>> Typical configurations often require >200KB for device state and VMDESC.
>>>>
>>>> Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
>>>> ---
>>>>    migration/migration.c | 4 +++-
>>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/migration/migration.c b/migration/migration.c
>>>> index ca8b97baa5..32b48fe9f5 100644
>>>> --- a/migration/migration.c
>>>> +++ b/migration/migration.c
>>>> @@ -3812,7 +3812,7 @@ static void *bg_migration_thread(void *opaque)
>>>>         * with vCPUs running and, finally, write stashed non-RAM part of
>>>>         * the vmstate from the buffer to the migration stream.
>>>>         */
>>>> -    s->bioc = qio_channel_buffer_new(128 * 1024);
>>>> +    s->bioc = qio_channel_buffer_new(512 * 1024);
>>>>        qio_channel_set_name(QIO_CHANNEL(s->bioc), "vmstate-buffer");
>>>>        fb = qemu_fopen_channel_output(QIO_CHANNEL(s->bioc));
>>>>        object_unref(OBJECT(s->bioc));
>>>> @@ -3866,6 +3866,8 @@ static void *bg_migration_thread(void *opaque)
>>>>        if (qemu_savevm_state_complete_precopy_non_iterable(fb, false, false)) {
>>>>            goto fail;
>>>>        }
>>>> +    qemu_fflush(fb);
>>> What will happen if the vmstates are bigger than 512KB?  Would the extra data
>>> be dropped?
>> No, the buffer shall be reallocated and the original content shall be kept.
> Oh right..
>
> Would you comment above qemu_fflush() about it (maybe also move it right before
> the call to qemu_put_buffer)?  Otherwise it indeed looks more like an
> optimization rather than a bugfix.

Agree, better to have a comment here.

> For the long term I think we'd better have a helper:
>
>          qemu_put_qio_channel_buffer(QEMUFile *file, QIOChannelBuffer *bioc)
>
> So as to hide this flush operation, which is tricky. We'll have two users so
> far:
>
>          bg_migration_completion
>          colo_do_checkpoint_transaction
>
> IMHO it'll be nicer if you'd do it in this patch altogether!
>
> Thanks,
>
Sorry, can't get the idea, what's wrong with the fix.


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