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
