Yoshiaki Tamura wrote: > Dor Laor wrote: > >On 04/21/2010 08:57 AM, Yoshiaki Tamura wrote: > >>Event tapping is the core component of Kemari, and it decides on which > >>event the > >>primary should synchronize with the secondary. The basic assumption > >>here is > >>that outgoing I/O operations are idempotent, which is usually true for > >>disk I/O > >>and reliable network protocols such as TCP. > > > >IMO any type of network even should be stalled too. What if the VM runs > >non tcp protocol and the packet that the master node sent reached some > >remote client and before the sync to the slave the master failed? > > In current implementation, it is actually stalling any type of network > that goes through virtio-net. > > However, if the application was using unreliable protocols, it should have > its own recovering mechanism, or it should be completely stateless. Even with unreliable protocols, if slave takeover causes the receiver to have received a packet that the sender _does not think it has ever sent_, expect some protocols to break. If the slave replaying master's behaviour since the last sync means it will definitely get into the same state of having sent the packet, that works out. But you still have to be careful that the other end's responses to that packet are not seen by the slave too early during that replay. Otherwise, for example, the slave may observe a TCP ACK to a packet that it hasn't yet sent, which is an error. About IP idempotency: In general, IP packets are allowed to be lost or duplicated in the network. All IP protocols should be prepared for that; it is a basic property. However there is one respect in which they're not idempotent: The TTL field should be decreased if packets are delayed. Packets should not appear to live in the network for longer than TTL seconds. If they do, some protocols (like TCP) can react to the delayed ones differently, such as sending a RST packet and breaking a connection. It is acceptable to reduce TTL faster than the minimum. After all, it is reduced by 1 on every forwarding hop, in addition to time delays. > I currently don't have good numbers that I can share right now. > Snapshots/sec depends on what kind of workload is running, and if the > guest was almost idle, there will be no snapshots in 5sec. On the other > hand, if the guest was running I/O intensive workloads (netperf, iozone > for example), there will be about 50 snapshots/sec. That is a really satisfying number, thank you :-) Without this work I wouldn't have imagined that synchronised machines could work with such a low transaction rate. -- Jamie -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
