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Re: [Qemu-devel] [PATCH v6 1/2] mirror: Rewrite mirror_iteration
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH v6 1/2] mirror: Rewrite mirror_iteration |
Date: |
Fri, 18 Dec 2015 17:41:45 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 |
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 <address@hidden>
>
> ---
> 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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- Re: [Qemu-devel] [PATCH v6 1/2] mirror: Rewrite mirror_iteration,
Max Reitz <=