On 20.11.2015 11:12, Fam Zheng wrote: > The "pnum < nb_sectors" condition in deciding whether to actually copy > data is unnecessarily strict, and the qiov initialization is > unnecessarily for bdrv_aio_write_zeroes and bdrv_aio_discard. > > Rewrite mirror_iteration to fix both flaws. > > Signed-off-by: Fam Zheng <f...@redhat.com> > > --- > v5: Address Max's review comments: > - Fix parameter name of mirror_do_read(). > - Simplify the buffer waiting loop in mirror_do_read. > - Don't skip next dirty chunk when collecting consective dirty > chunks. > - Check sector range when collecting consective dirty chunks. > - Don't misuse a negative return value of > bdrv_get_block_status_above. > --- > block/mirror.c | 307 > +++++++++++++++++++++++++++++++++++---------------------- > 1 file changed, 187 insertions(+), 120 deletions(-) > > diff --git a/block/mirror.c b/block/mirror.c > index 52c9abf..ff8149d 100644 > --- a/block/mirror.c > +++ b/block/mirror.c > @@ -45,7 +45,6 @@ typedef struct MirrorBlockJob { > BlockdevOnError on_source_error, on_target_error; > bool synced; > bool should_complete; > - int64_t sector_num; > int64_t granularity; > size_t buf_size; > int64_t bdev_length; > @@ -157,113 +156,76 @@ static void mirror_read_complete(void *opaque, int ret) > mirror_write_complete, op); > } > > -static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s) > +/* Round sector_num and/or nb_sectors to target cluster if COW is needed, and > + * return the offset of the adjusted ending sector against > + * sector_num + nb_sectors. */ > +static int mirror_cow_align(MirrorBlockJob *s, > + int64_t *sector_num, > + int *nb_sectors) > +{ > + bool head_need_cow, tail_need_cow; > + int diff = 0; > + int sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS; > + > + head_need_cow = !test_bit(*sector_num / sectors_per_chunk, > s->cow_bitmap); > + tail_need_cow = !test_bit((*sector_num + *nb_sectors) / > sectors_per_chunk,
Should this be (*sector_num + *nb_sectors - 1) so that we actually check the last chunk of the sector range (instead of the next chunk if *sector_num + *nb_sectors is divisible by sectors_per_chunk). > + s->cow_bitmap); > + if (head_need_cow || tail_need_cow) { > + int64_t rounded_sector_num; > + int rounded_nb_sectors; > + bdrv_round_to_clusters(s->target, *sector_num, *nb_sectors, > + &rounded_sector_num, &rounded_nb_sectors); > + assert(*sector_num >= rounded_sector_num); > + assert(rounded_nb_sectors >= *nb_sectors); You could move these assertions into the following conditional blocks, that would make more sense to me: > + if (tail_need_cow) { > + int diff = rounded_sector_num + rounded_nb_sectors > + - (*sector_num + *nb_sectors); assert(diff >= 0); Also, I don't like shadowing of variables very much. > + *nb_sectors += diff; > + } > + if (head_need_cow) { > + int diff = *sector_num - rounded_sector_num; assert(diff >= 0); > + *sector_num = rounded_sector_num; > + *nb_sectors += diff; > + } > + } > + return diff; diff is always 0. (This is why I don't like shadowing of variables very much.) > +} > + > +/* Submit async read while handling COW. > + * Returns: nb_sectors if no alignment is necessary, or > + * (new_end - sector_num) if tail is rounded up or down due to > + * alignment or buffer limit. > + */ > +static int mirror_do_read(MirrorBlockJob *s, int64_t sector_num, > + int nb_sectors) > { > BlockDriverState *source = s->common.bs; > - int nb_sectors, sectors_per_chunk, nb_chunks; > - int64_t end, sector_num, next_chunk, next_sector, hbitmap_next_sector; > - uint64_t delay_ns = 0; > + int sectors_per_chunk, nb_chunks; > + int ret = nb_sectors; > MirrorOp *op; > - int pnum; > - int64_t ret; > > - s->sector_num = hbitmap_iter_next(&s->hbi); > - if (s->sector_num < 0) { > - bdrv_dirty_iter_init(s->dirty_bitmap, &s->hbi); > - s->sector_num = hbitmap_iter_next(&s->hbi); > - trace_mirror_restart_iter(s, bdrv_get_dirty_count(s->dirty_bitmap)); > - assert(s->sector_num >= 0); > - } > - > - hbitmap_next_sector = s->sector_num; > - sector_num = s->sector_num; > sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS; > - end = s->bdev_length / BDRV_SECTOR_SIZE; > > - /* Extend the QEMUIOVector to include all adjacent blocks that will > - * be copied in this operation. > - * > - * We have to do this if we have no backing file yet in the destination, > - * and the cluster size is very large. Then we need to do COW ourselves. > - * The first time a cluster is copied, copy it entirely. Note that, > - * because both the granularity and the cluster size are powers of two, > - * the number of sectors to copy cannot exceed one cluster. > - * > - * We also want to extend the QEMUIOVector to include more adjacent > - * dirty blocks if possible, to limit the number of I/O operations and > - * run efficiently even with a small granularity. > - */ > - nb_chunks = 0; > - nb_sectors = 0; > - next_sector = sector_num; > - next_chunk = sector_num / sectors_per_chunk; > + if (s->cow_bitmap) { > + ret += mirror_cow_align(s, §or_num, &nb_sectors); mirror_cow_align() always returns 0, but I assume it is supposed to return the difference of nb_sectors before and after the call (in which case this line is correct). > + } > + /* We can only handle as much as buf_size at a time. */ > + nb_sectors = MIN(s->buf_size >> BDRV_SECTOR_BITS, nb_sectors); > + assert(nb_sectors); Maybe we should move these three lines before the mirror_cow_align() call. I don't think it will make a difference in practice, but it seems cleaner to me that way. > + /* The sector range must meet granularity because: > + * 1) Caller passes in aligned values; > + * 2) mirror_cow_align is used only when target cluster is larger. */ > + assert(!(nb_sectors % sectors_per_chunk)); > + assert(!(sector_num % sectors_per_chunk)); > + nb_chunks = nb_sectors / sectors_per_chunk; [...] > +static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s) > +{ [...] > + while (nb_chunks > 0 && sector_num < end) { > + int ret; > + int io_sectors; > + enum MirrorMethod { > + MIRROR_METHOD_COPY, > + MIRROR_METHOD_ZERO, > + MIRROR_METHOD_DISCARD > + } mirror_method = MIRROR_METHOD_COPY; > + > + assert(!(sector_num % sectors_per_chunk)); > + ret = bdrv_get_block_status_above(source, NULL, sector_num, > + nb_chunks * sectors_per_chunk, > + &io_sectors); > + if (ret < 0) { > + io_sectors = nb_chunks * sectors_per_chunk; > + } > + > + io_sectors -= io_sectors % sectors_per_chunk; > + if (io_sectors < sectors_per_chunk) { > + io_sectors = sectors_per_chunk; > + } else if (ret > 0 && !(ret & BDRV_BLOCK_DATA)) { Why > 0 and not >= 0? Max > + int64_t target_sector_num; > + int target_nb_sectors; > + bdrv_round_to_clusters(s->target, sector_num, io_sectors, > + &target_sector_num, &target_nb_sectors); > + if (target_sector_num == sector_num && > + target_nb_sectors == io_sectors) { > + mirror_method = ret & BDRV_BLOCK_ZERO ? > + MIRROR_METHOD_ZERO : > + MIRROR_METHOD_DISCARD; > + } > + } > + > + switch (mirror_method) { > + case MIRROR_METHOD_COPY: > + io_sectors = mirror_do_read(s, sector_num, io_sectors); > + break; > + case MIRROR_METHOD_ZERO: > + mirror_do_zero_or_discard(s, sector_num, io_sectors, false); > + break; > + case MIRROR_METHOD_DISCARD: > + mirror_do_zero_or_discard(s, sector_num, io_sectors, true); > + break; > + default: > + abort(); > + } > + assert(io_sectors); > + sector_num += io_sectors; > + nb_chunks -= io_sectors / sectors_per_chunk; > + delay_ns += ratelimit_calculate_delay(&s->limit, io_sectors); > } > return delay_ns; > } >
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