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
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 <stefa...@linux.vnet.ibm.com>
---
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
