[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 4/9] mirror: efficiently zero out target
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH 4/9] mirror: efficiently zero out target |
Date: |
Tue, 14 Jun 2016 21:00:27 -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:
> With a bdrv_co_write_zeroes method on a target BDS zeroes will not be placed
> into the wire. Thus the target could be very efficiently zeroed out. This
> is should be done with the largest chunk possible.
>
> This improves the performance of the live migration of the empty disk by
> 150 times if NBD supports write_zeroes.
>
> 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 | 32 +++++++++++++++++++++++++++++++-
> 1 file changed, 31 insertions(+), 1 deletion(-)
>
> diff --git a/block/mirror.c b/block/mirror.c
> index c7b3639..c2f8773 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -21,6 +21,7 @@
> #include "qemu/ratelimit.h"
> #include "qemu/bitmap.h"
>
> +#define MIRROR_ZERO_CHUNK (3u << (29 - BDRV_SECTOR_BITS)) /* 1.5 Gb */
Probably nicer to track this in bytes. And do you really want a
hard-coded arbitrary limit, or is it better to live with
MIN_NON_ZERO(target_bs->bl.max_pwrite_zeroes, INT_MAX)?
> @@ -512,7 +513,8 @@ static int mirror_dirty_init(MirrorBlockJob *s)
>
> end = s->bdev_length / BDRV_SECTOR_SIZE;
>
> - if (base == NULL && !bdrv_has_zero_init(target_bs)) {
> + if (base == NULL && !bdrv_has_zero_init(target_bs) &&
> + target_bs->drv->bdrv_co_write_zeroes == NULL) {
Indentation is off, although if checkpatch.pl doesn't complain I guess
it doesn't matter that much.
Why should you care whether the target_bs->drv implements a callback?
Can't you just rely on the normal bdrv_*() functions to do the dirty
work of picking the most efficient implementation without you having to
bypass the block layer? In fact, isn't that the whole goal of
bdrv_make_zero() - why not call that instead of reimplementing it?
Patch needs rebasing - we've redone this into bdrv_co_pwrite_zeroes and
a byte interface, since upstream commit c1499a5e.
> bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, end);
> return 0;
> }
> @@ -546,6 +548,34 @@ static int mirror_dirty_init(MirrorBlockJob *s)
> }
> sector_num += n;
> }
> +
> + if (base != NULL || bdrv_has_zero_init(target_bs)) {
You're now repeating the conditional that used to be 'bool
mark_all_dirty' (well, this is !mark_all_dirty); is it worth keeping the
simpler bool around?
> + /* no need to zero out entire disk */
> + return 0;
> + }
> +
> + for (sector_num = 0; sector_num < end; ) {
> + int nb_sectors = MIN(MIRROR_ZERO_CHUNK, end - sector_num);
Why limit yourself to 1.5G? It's either too small for what you can
really do, or too large for what the device permits. See my above
comment about MIN_NON_ZERO.
> + int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> +
> + if (now - last_pause_ns > SLICE_TIME) {
> + last_pause_ns = now;
> + block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, 0);
> + }
> +
> + if (block_job_is_cancelled(&s->common)) {
> + return -EINTR;
> + }
> +
> + if (s->in_flight == MAX_IN_FLIGHT) {
> + trace_mirror_yield(s, s->in_flight, s->buf_free_count, -1);
> + mirror_wait_for_io(s);
> + continue;
> + }
Hmm - I guess your mirror yield points are why you couldn't just
directly use bdrv_make_zero(); but is that something where some code
refactoring can share more code rather than duplicating it?
> +
> + mirror_do_zero_or_discard(s, sector_num, nb_sectors, false);
> + sector_num += nb_sectors;
> + }
> return 0;
> }
>
>
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature
[Qemu-devel] [PATCH 2/9] mirror: create mirror_dirty_init helper for mirror_run, Denis V. Lunev, 2016/06/14
[Qemu-devel] [PATCH 6/9] block: pass qiov into before_write notifier, Denis V. Lunev, 2016/06/14