Re: [Xen-devel] [PATCH v7 12/18] tools/libx{l, c}: add back channel to libxc

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Feb 04, 2016 at 01:28:14PM +0800, Wen Congyang wrote:
> On 02/04/2016 03:40 AM, Wei Liu wrote:
> > On Fri, Jan 29, 2016 at 01:27:28PM +0800, Wen Congyang wrote:
> >> In COLO mode, both VMs are running, and are considered in sync if the
> >> visible network traffic is identical.  After some time, they fall out of
> >> sync.
> >>
> >> At this point, the two VMs have definitely diverged.  Lets call the
> >> primary dirty bitmap set A, while the secondary dirty bitmap set B.
> >>
> >> Sets A and B are different.
> >>
> >> Under normal migration, the page data for set A will be sent from the
> >> primary to the secondary.
> >>
> >> However, the set difference B - A (the one in B but not in A, lets
> >> call this C) is out-of-date on the secondary (with respect to the
> >> primary) and will not be sent by the primary (to secondary), as it
> >> was not memory dirtied by the primary. The secondary needs C page data
> >> to reconstruct an exact copy of the primary at the checkpoint.
> >>
> >> The secondary cannot calculate C as it doesn't know A.  Instead, the
> >> secondary must send B to the primary, at which point the primary
> >> calculates the union of A and B (lets call this D) which is all the
> >> pages dirtied by both the primary and the secondary, and sends all page
> >> data covered by D.
> >>
> >> In the general case, D is a superset of both A and B.  Without the
> >> backchannel dirty bitmap, a COLO checkpoint can't reconstruct a valid
> >> copy of the primary.
> >>
> >> We transfer the dirty bitmap on libxc side, so we need to introduce back
> >> channel to libxc.
> >>
> >> Note: it is different from the paper. We change the original design to
> >> the current one, according to our following concerns:
> >> 1. The original design needs extra memory on Secondary host. When there's
> >>    multiple backups on one host, the memory cost is high.
> >> 2. The memory cache code will be another 1k+, it will make the review
> >>    more time consuming.
> >>
> >> Note: the back channel will be used in the patch
> > 
> > "will not be used" ?
> > 
> > I don't see any read / write to the newly introduced fd.
> 
> It is used in COLO series.
> 
> Some patches in this series just introduce an API. Thess APIs will be used
> in COLO series. Do you mean that these patches should be put in COLO series?
> If so, I will check all patches.
> 

Fine by me.

Wei.

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