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Re: [Qemu-block] [PATCH 3/9] mirror: optimize dirty bitmap filling in mi

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 3/9] mirror: optimize dirty bitmap filling in mirror_run a bit
Date: Tue, 14 Jun 2016 20:36:53 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/14/2016 09:25 AM, Denis V. Lunev wrote:
> There is no need to scan allocation tables if we have mark_all_dirty flag
> set. Just mark it all dirty.
> Signed-off-by: Denis V. Lunev <address@hidden>
> Reviewed-by: Vladimir Sementsov-Ogievskiy<address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Fam Zheng <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Max Reitz <address@hidden>
> CC: Jeff Cody <address@hidden>
> CC: Eric Blake <address@hidden>
> ---
>  block/mirror.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> diff --git a/block/mirror.c b/block/mirror.c
> index 797659d..c7b3639 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -507,12 +507,16 @@ static int mirror_dirty_init(MirrorBlockJob *s)
>      BlockDriverState *base = s->base;
>      BlockDriverState *bs = blk_bs(s->common.blk);
>      BlockDriverState *target_bs = blk_bs(s->target);
> -    bool mark_all_dirty = base == NULL && !bdrv_has_zero_init(target_bs);
>      uint64_t last_pause_ns;
>      int ret, n;
>      end = s->bdev_length / BDRV_SECTOR_SIZE;
> +    if (base == NULL && !bdrv_has_zero_init(target_bs)) {
> +        bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, end);

Won't work as written.  'end' is 64 bits, but bdrv_set_dirty_bitmap() is
limited to a 32-bit sector count.  Might be first worth updating
bdrv_set_dirty_bitmap() and friends to be byte-based rather than
sector-based (but still tracking a sector per bit, obviously), as well
as expand it to operate on 64-bit sizes rather than 32-bit.

I'm also worried slightly that the existing code repeated things in a
loop, and therefore had pause points every iteration and could thus
remain responsive to an early cancel.  But doing the entire operation in
one chunk (assuming you fix bitmap code to handle a 64-bit size) may end
up running for so long without interruption that you lose the benefits
of an early interruption that you have by virtue of a 32-bit limit.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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