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

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

[-- Attachment #1: Type: text/plain, Size: 1831 bytes --]

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.

> In that case, I'm wondering whether we'll need a qemu_file_get_error() after
> the flush to detect it, and whether we need to retry with a bigger buffer size?
>
> Thanks,
>
I think for QIOBufferChannel we don't need that, as I can see from the buffer channel
implementation.


[-- Attachment #2: Type: text/html, Size: 2457 bytes --]