Am 08.02.2012 10:47, schrieb Ori Mamluk: > On 08/02/2012 10:55, Kevin Wolf wrote: >> Am 08.02.2012 07:10, schrieb Ori Mamluk: >>> On 07/02/2012 17:47, Paolo Bonzini wrote: >>>> >>>> Why can't QEMU itself stream the full volume in the background, and >>>> send that together with any new I/O? Is it because the rephub knows >>>> which parts are out-of-date and need recovery? In that case, as a >>>> first approximation the rephub can pass the sector at which streaming >>>> should start. >>> Yes - it's because rephub knows. The parts that need recovery may be a >>> series of random IOs that were lost because of a network outage >>> somewhere along the replication pipe. >>> Easy to think of it as a bitmap holding the not-yet-replicated IOs. The >>> rephub occasionally reads those areas to 'sync' them, so in effect the >>> rephub needs read access - it's not really to trigger streaming from an >>> offset. >> So how does the rephub know which areas were touched by lost requests? >> Isn't qemu the only one who could know what it sent? >> >> Kevin > You're right. Currently only Qemu knows.
How could it change later on? If the network is down, qemu can't communicate it to anyone else, so it stays the only one who knows. > The problem is that if we move the responsibility to a layer below Qemu > - then rephub will never know. > Our (Zerto's) solution for vmware has a different design, but it has 3 > parts relevant to this discussion: > 1. Tapping to protected writes / read protected volume > 2. Maintain a bitmap > 3. Provide cross-VM consistency for recovery. > > I want to simplify our design by taking it one step at a time. > My first goal for Qemu is to have only step 1 - meaning tap all > protected writes, and be able to read. > I think it will be simpler for all of us to complete that first, and it > provides a basic ability (though not optimal) for protection and recovery. > > I think using an external streaming mechanism will make the next stages > impossible. Well, then we need to discuss all stages now. If you tell only part of what you're going to do, you'll get a design that will only work for part of what you need. Kevin
