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Re: [Qemu-devel] [PATCH] A small patch to introduce stop conditions to t

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] A small patch to introduce stop conditions to the live migration.
Date: Wed, 14 Sep 2011 10:45:19 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110516 Lightning/1.0b2 Thunderbird/3.1.10

On 09/14/2011 08:18 AM, Thomas Treutner wrote:
Currently, it is possible that a live migration never finishes, when the dirty 
page rate is high compared to the scan/transfer rate. The exact values for 
there should be *some* limit to force the final iteration of a live migration 
that does not converge.

No, there shouldn't be.

A management app can always stop a guest to force convergence. If you make migration have unbounded downtime by default then you're making migration unsafe for smarter consumers.

You can already set things like maximum downtime to force convergence. If you wanted to have some logic like an exponentially increasing maximum downtime given a fixed timeout, that would be okay provided it was an optional feature.

So for instance, you could do something like:

downtime: defaults to 30ms

(qemu) migrate_set_convergence_timeout 60 # begin to enforce convergence after 1 minute

At 1 minutes, downtime goes to 60ms, at 2 minutes it goes to 120ms, at 3 minutes it goes to 240ms, 4 minutes it goes to 480ms, 5 minutes it goes to 1.06s, 6 minutes it goes to 2s, etc.


Anthony Liguori

  arch_init.c |   10 +++++++++-
  1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index 4486925..57fcb1e 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -89,6 +89,9 @@ const uint32_t arch_type = QEMU_ARCH;
  #define RAM_SAVE_FLAG_EOS      0x10

  static int is_dup_page(uint8_t *page, uint8_t ch)
      uint32_t val = ch<<  24 | ch<<  16 | ch<<  8 | ch;
@@ -107,6 +110,8 @@ static int is_dup_page(uint8_t *page, uint8_t ch)
  static RAMBlock *last_block;
  static ram_addr_t last_offset;

+static int numberFullMemoryIterations = 0;
  static int ram_save_block(QEMUFile *f)
      RAMBlock *block = last_block;
@@ -158,7 +163,10 @@ static int ram_save_block(QEMUFile *f)
              offset = 0;
              block = QLIST_NEXT(block, next);
              if (!block)
+            {
+                numberFullMemoryIterations++;
                  block = QLIST_FIRST(&ram_list.blocks);
+            }

          current_addr = block->offset + offset;
@@ -295,7 +303,7 @@ int ram_save_live(Monitor *mon, QEMUFile *f, int stage, 
void *opaque)

      expected_time = ram_save_remaining() * TARGET_PAGE_SIZE / bwidth;

-    return (stage == 2)&&  (expected_time<= migrate_max_downtime());
+    return (stage == 2)&&  ((expected_time<= migrate_max_downtime() || 
(numberFullMemoryIterations == MAX_MEMORY_ITERATIONS) || (bytes_transferred>  

  static inline void *host_from_stream_offset(QEMUFile *f,

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