[Qemu-devel] [PATCH RFC 0/4] Curling: KVM Fault Tolerance

[Jules Wang]

The goal of Curling(sports) is to provide a fault tolerant mechanism for KVM,
so that in the event of a hardware failure, the virtual machine fails over to
the backup in a way that is completely transparent to the guest operating 
system.

Our goal is exactly the same as the goal of Kemari, by which Curling is
inspired. However, Curling is simpler than Kemari(too simple, I afraid):

* By leveraging live migration feature, we do endless live migrations between
the sender and receiver, so the two virtual machines are synchronized.

* The receiver does not load vm state once the migration begins, instead, it
perfetches one whole migration data into a buffer, then loads vm state from that
buffer afterwards. This "all or nothing" approach prevents the
broken-in-the-middle problem Kemari has.

* The sender sleeps a little while after each migration, to ease the performance
penalty entailed by vm_stop and iothread locks. This is a tradeoff between
performance and accuracy.

Usage:
The steps of curling are the same as the steps of live migration except the
following:
1. Start the receiver vm with -incoming curling:tcp:<address>:<port>
2. Start ft in the qemu monitor of sender vm by following cmdline:
   > migrate_set_speed  <full bandwidth>
   > migrate curling:tcp:<address>:<port>
3. Connect to the receiver vm by vnc or spice. The screen of the vm is displayed
when curling is ready.
4. Now, the sender vm is protected by ft, When it encounters a failure,
the failover kicks in.

Problems to be discussed:
1. When the receiver is prefectching data, how does it know where is the EOF of
one migration?

Currently, we use a magic number 0xfeedcafe to indicate the EOF.
Any better solutions?

2. How to reduce the overhead entailed by vm_stop and iothread locks?

Any solutions other than sleeping?

--

Jules Wang (4):
  Curling: add doc
  Curling: cmdline interface
  Curling: the sender
  Curling: the receiver

 arch_init.c                   |  18 +++--
 docs/curling.txt              |  52 ++++++++++++++
 include/migration/migration.h |   2 +
 include/migration/qemu-file.h |   1 +
 include/sysemu/sysemu.h       |   1 +
 migration.c                   |  61 ++++++++++++++--
 savevm.c                      | 158 ++++++++++++++++++++++++++++++++++++++++--
 7 files changed, 277 insertions(+), 16 deletions(-)
 create mode 100644 docs/curling.txt

-- 
1.8.0.1