Re: [Qemu-devel] block replication

[Xie Changlong <xiechanglong.d@gmail.com>]

在 8/10/2017 8:26 PM, Vladimir Sementsov-Ogievskiy 写道:
> 09.08.2017 17:11, Vladimir Sementsov-Ogievskiy wrote:
>> Hi Wen!
>>
>> I'm trying to understand block/replication code and have a question.
>>
>> Why should we block the region from intersecting cow requests when 
>> read? If I understand correctly
>>
>> regardless of writes to the secondary-disk we have consistent view of 
>> it through hidden-disk:
>>
>> Even if we are intersecting with some writes to secondary-disk (and 
>> corresponding cow-requests), the
>>
>> data in secondary disk will not be updated until backed up to 
>> hidden-disk, therefore, for read we have two
>>
>> options:
>>
>> 1. read old data from secondary-disk (unallocated region in 
>> hidden-disk means data in secondary-disk is not updated yet)
>>
>> 2. read backed-up data from hidden-disk (data in secondary-disk may be 
>> already updated but we don't care)
>>
>> (the whole region to read may consists of parts, corresponding to 1 or 
>> 2, but this should be ok too)
>>
>> Where am I wrong?
> 
> Ok, now I think this is needed to prevent intersecting of writes and 
> reads on hidden-disk. If it so, I think it is better to use serializing 

Hi, Vladimir. Pls refer 
https://lists.nongnu.org/archive/html/qemu-devel/2016-05/msg03025.html

BTW, wen's email has changed, also CC Zhang Hailiang, Zhang Chen, Li Zhijian

> requests
> mechanism (just serialize all requests on hidden-disk, and on write wait 
> for all intersecting serializing requests, on read wait for intersecting 
> serializing writes) - it may require additional
> option for BlockDriverState, but it is more generic and more clear than 
> export internal backup things to lock disk region. This also can be 
> reused for image-fleecing scheme
> (which is based on same pattern  [active-disk is backing for temp-disk, 
> backup sync=none from active to temp, read from temp])
> 
>>
>>
>> ======
>>
>> static coroutine_fn int replication_co_readv(BlockDriverState *bs,
>>                                              int64_t sector_num,
>>                                              int remaining_sectors,
>>                                              QEMUIOVector *qiov)
>> {
>>     BDRVReplicationState *s = bs->opaque;
>>     BdrvChild *child = s->secondary_disk;
>>     BlockJob *job = NULL;
>>     CowRequest req;
>>     int ret;
>>
>>     if (s->mode == REPLICATION_MODE_PRIMARY) {
>>         /* We only use it to forward primary write requests */
>>         return -EIO;
>> }
>>
>>     ret = replication_get_io_status(s);
>>     if (ret < 0) {
>>         return ret;
>> }
>>
>>     if (child && child->bs) {
>>         job = child->bs->job;
>> }
>>
>>     if (job) {
>>         uint64_t remaining_bytes = remaining_sectors * BDRV_SECTOR_SIZE;
>>
>> backup_wait_for_overlapping_requests(child->bs->job,
>>                                              sector_num * 
>> BDRV_SECTOR_SIZE,
>> remaining_bytes);
>>         backup_cow_request_begin(&req, child->bs->job,
>>                                  sector_num * BDRV_SECTOR_SIZE,
>> remaining_bytes);
>>         ret = bdrv_co_readv(bs->file, sector_num, remaining_sectors,
>> qiov);
>> backup_cow_request_end(&req);
>>         goto out;
>> }
>>
>>     ret = bdrv_co_readv(bs->file, sector_num, remaining_sectors, qiov);
>> out:
>>     return replication_return_value(s, ret);
>> }
>>
> 

-- 
Thanks
     -Xie