Re: [Qemu-devel] Re: Strategic decision: COW format

[Stefan Hajnoczi]

On Fri, Feb 25, 2011 at 11:20 AM, Pavel Dovgaluk
<address@hidden> wrote:
>
>> On 02/23/2011 05:50 PM, Anthony Liguori wrote:
>> >> I still don't see.  What would you do with thousands of checkpoints?
>> >
>> >
>> > For reverse debugging, if you store checkpoints at a rate of save,
>> > every 10ms, and then degrade to storing every 100ms after 1 second,
>> > etc. you'll have quite a large number of snapshots pretty quickly.
>> > The idea of snapshotting with reverse debugging is that instead of
>> > undoing every instruction, you can revert to the snapshot before, and
>> > then replay the instruction stream until you get to the desired point
>> > in time.
>>
>> You cannot replay the instruction stream since inputs (interrupts, rdtsc
>> or other timers, I/O) will be different.  You need Kemari for this.
>
>  I've created the technology for replaying instruction stream and all of the
> inputs. This technology is similar to deterministic replay in VMWare.
>  Now I need something to save machine state in many checkpoints to
> implement reverse debugging.
>  I think COW2 may be useful for it (or I should create something like this).

Or the BTRFS_IOC_CLONE ioctl on the btrfs filesystem.  You can
copy-on-write clone a file using it.

Stefan