Re: [Xen-devel] [PATCH Remus v5 2/2] libxc/restore: implement Remus checkpointed restore

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-05-15 at 17:19 +0800, Yang Hongyang wrote:
> 
> On 05/15/2015 05:09 PM, Ian Campbell wrote:
> > On Fri, 2015-05-15 at 09:32 +0800, Yang Hongyang wrote:
> >>
> >> On 05/14/2015 09:05 PM, Ian Campbell wrote:
> >>> On Thu, 2015-05-14 at 18:06 +0800, Yang Hongyang wrote:
> >>>> With Remus, the restore flow should be:
> >>>> the first full migration stream -> { periodically restore stream }
> >>>>
> >>>> Signed-off-by: Yang Hongyang <yanghy@xxxxxxxxxxxxxx>
> >>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >>>> CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> >>>> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> >>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> >>>> ---
> >>>>    tools/libxc/xc_sr_common.h  |  14 ++++++
> >>>>    tools/libxc/xc_sr_restore.c | 113 
> >>>> ++++++++++++++++++++++++++++++++++++++++----
> >>>>    2 files changed, 117 insertions(+), 10 deletions(-)
> >>>>
> >>>> diff --git a/tools/libxc/xc_sr_common.h b/tools/libxc/xc_sr_common.h
> >>>> index f8121e7..3bf27f1 100644
> >>>> --- a/tools/libxc/xc_sr_common.h
> >>>> +++ b/tools/libxc/xc_sr_common.h
> >>>> @@ -208,6 +208,20 @@ struct xc_sr_context
> >>>>                /* Plain VM, or checkpoints over time. */
> >>>>                bool checkpointed;
> >>>>
> >>>> +            /* Currently buffering records between a checkpoint */
> >>>> +            bool buffer_all_records;
> >>>> +
> >>>> +/*
> >>>> + * With Remus, we buffer the records sent by the primary at checkpoint,
> >>>> + * in case the primary will fail, we can recover from the last
> >>>> + * checkpoint state.
> >>>> + * This should be enough because primary only send dirty pages at
> >>>> + * checkpoint.
> >>>
> >>> I'm not sure how it then follows that 1024 buffers is guaranteed to be
> >>> enough, unless there is something on the sending side arranging it to be
> >>> so?
> >>
> >> There are only few records at every checkpoint in my test, mostly under 10,
> >> probably because I don't do much operations in the Guest. I thought This 
> >> limit
> >> can be adjusted later by further testing.
> >
> > For some reason I thought these buffers included the page data, is that
> > not true? I was expecting the bulk of the records to be dirty page data.
> 
> The page data is not stored in this buffer, but it's pointer stored in
> this buffer(rec->data). This buffer is the bulk of the struct xc_sr_record.

OK, so there are (approximately) as many xc_sr_records as there are
buffered dirty pages? I'd expect this would easily reach 1024 in some
circumstances (e..g run a fork bomb in the domain or something)

> >> Since you and Andy both have doubts on this, I have to reconsider on this,
> >> perhaps there should be no limit. Even if the 1024 limit works for
> >> most of the cases, there might be cases that exceed the limit. So I will
> >> add another member 'allocated_rec_num' in the context, when the
> >> 'buffered_rec_num' exceed the 'allocated_rec_num', I will reallocate the 
> >> buffer.
> >> The initial buffer size will be 1024 records which will work for most 
> >> cases.
> >
> > That seems easy enough to be worth doing even if I was wrong about paged
> > data.
> 
> done.
> 
> >
> >
> > .
> >
> 



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