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Date: Fri, 25 Feb 2011 13:22:59 +0000
Message-ID: <AANLkTikKC3bA54he3Q_1AwAOZaDwsrfjQb4fhgKRZZGg@mail.gmail.com>
Subject: Re: [Qemu-devel] Re: Strategic decision: COW format
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Pavel Dovgaluk <Pavel.Dovgaluk@ispras.ru>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Markus Armbruster <armbru@redhat.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Chunqiang Tang <ctang@us.ibm.com>

On Fri, Feb 25, 2011 at 11:20 AM, Pavel Dovgaluk
<Pavel.Dovgaluk@ispras.ru> wrote:
>
>> On 02/23/2011 05:50 PM, Anthony Liguori wrote:
>> >> I still don't see. =A0What 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. =A0You need Kemari for this.
>
> =A0I've created the technology for replaying instruction stream and all o=
f the
> inputs. This technology is similar to deterministic replay in VMWare.
> =A0Now I need something to save machine state in many checkpoints to
> implement reverse debugging.
> =A0I think COW2 may be useful for it (or I should create something like t=
his).

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

Stefan