Re: [Xen-devel] [RFC V5 2/5] Libxl Domain Snapshot API Design

["Bamvor Jian Zhang" <bjzhang@xxxxxxxx>]

Hi, Ian

> Ian Campbell writes ("Re: [RFC V5 2/5] Libxl Domain Snapshot API Design"):
> > On Mon, 2014-07-07 at 15:46 +0800, Bamvor Jian Zhang wrote:
> ...
> > >         ("memory",        string),       /* The path to save domain
> > >                                           * memory image. 'empty' means
> > >                                           * it is a disk-only snapshot.
> > >                                           * note that "yes" or "no" is
> > >                                           * not allowed, this is different
> > >                                           * from xl.snapshot.pod.5
>
> As I wrote on IRC:
sorry, i forget logging out my irc when i leave company.
>
> I still don't really understand how the distinction between snapshots
> with memory=yes and memory=no is supposed to operate.  Is it that if
> you take a snapshot of a running domain with memory=no, and then later
> resume it, it is as if the guest had crashed at the time of the
> snapshot ?  (How does this interact with disk IO barriers?)
yes, as you said, the domain may crash after disk stae resume if
there is no memory state resume.
basically, this is compatible with libvirt domain snapshot, ref[1].
after read the libvirt and qemu code again, i found that there is a cpu
stop qmp before create disk only snapshot when domain is running. such
qmp will call bdrv_drain_all and bdrv_flush_all for flush all data to
disk.
compare to libxl, in my current code, i call libxl_domain_pause before
creating disk only snapshot. it seems that libxl_domain_pause cause the
vcpu stop through hypercall. and i do not see the similar flush date to
disk operation along with vcpus stop.
how should i deal with it? could i call stop cpu qmp after
libxl_domain_pause in order to flush data to disk?
>
> I think the semantics should be spelled out somewhere.
>
> (Also I assume that a memory snapshot isn't available unless there's a
> running domain, but that also needs to be specified.)
yes, definitely. i will add this to the document.

regards

bamvor

[1] http://libvirt.org/formatsnapshot.html
>
> Thanks,
> Ian.


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