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Re: [Qemu-devel] [PATCH] qed: replace vm_clock with rt_clock for qemu-to
From: |
Zhi Yong Wu |
Subject: |
Re: [Qemu-devel] [PATCH] qed: replace vm_clock with rt_clock for qemu-tool compatibility |
Date: |
Mon, 27 Feb 2012 22:20:05 +0800 |
On Sun, Feb 26, 2012 at 10:55 PM, Stefan Hajnoczi
<address@hidden> wrote:
> The QED dirty bit timer marks the file clean after allocating writes
> have drained. This is cheaper than clearing/setting the dirty bit on
> each allocating write because the timer introduces a grace period which
> can be extended if more allocating writes arrive.
>
> The vm_clock was used in an attempt to prevent modifying the image file
Why can vm_clock do this work but other clock can not? If you're
available, can you elaborate what the difference among the three
clocks is? such as vm_clock, rt_clock, and host_clock.
> when live migration has stopped the VM. Unfortunately vm_clock is
> unavailable in the qemu-tool environment and will abort(3)!
>
> Since QED currently does not support live migration, just replace
> vm_clock with rt_clock and add comments explaining the migration
> blocker.
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> Zhi Yong: This patch is needed in addition to the qemu_init_main_loop()
> patches
> you sent recently. Without this patch QED may read the vm_clock, which calls
> abort(3) in qemu-tool.c. Together, our patches make QED work again in
> qemu-img
> and qemu-io.
>
> block/qed.c | 16 +++++++++++-----
> 1 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/block/qed.c b/block/qed.c
> index a041d31..fdb90e3 100644
> --- a/block/qed.c
> +++ b/block/qed.c
> @@ -353,10 +353,7 @@ static void qed_start_need_check_timer(BDRVQEDState *s)
> {
> trace_qed_start_need_check_timer(s);
>
> - /* Use vm_clock so we don't alter the image file while suspended for
> - * migration.
> - */
> - qemu_mod_timer(s->need_check_timer, qemu_get_clock_ns(vm_clock) +
> + qemu_mod_timer(s->need_check_timer, qemu_get_clock_ns(rt_clock) +
> get_ticks_per_sec() * QED_NEED_CHECK_TIMEOUT);
> }
>
> @@ -494,9 +491,18 @@ static int bdrv_qed_open(BlockDriverState *bs, int flags)
> }
> }
>
> - s->need_check_timer = qemu_new_timer_ns(vm_clock,
> + s->need_check_timer = qemu_new_timer_ns(rt_clock,
> qed_need_check_timer_cb, s);
>
> + /* There are two issues with live migration:
> + *
> + * 1. The destination will open the image file and see the dirty bit is
> + * set, causing it to "repair" the image while the source still has it
> + * open for writing.
> + *
> + * 2. The timer used for clearing the dirty bit uses rt_clock and can in
> + * theory fire when the VM is not running during migration.
> + */
> error_set(&s->migration_blocker,
> QERR_BLOCK_FORMAT_FEATURE_NOT_SUPPORTED,
> "qed", bs->device_name, "live migration");
> --
> 1.7.9
>
>
--
Regards,
Zhi Yong Wu