[libvirt] [PATCH] docs: Add detailed notes snapshots, blockcommit, blockpull

[Gao Yongwei]

> > +
> > +The above figure is another representation of reducing the disk image
> chain
> > +using blockcommit. Data from Snap-1, Snap-2, Snap-3 are
> merged(/committed)
> > +into RootBase, & now the current 'Active' image now pointing to
> 'RootBase' as its
> > +backing file(instead of Snap-3, which was the case *before*
> blockcommit). Note
> > +that, now intermediate images Snap-1, Snap-1, Snap-3 will be
> invalidated(as they were
>

......................................................Snap-1,Snap-2,........

 > +dependent on a particular state of RootBase).
> > +
> > +blockpull
> ....................................................................
> > +**Figure-5**
> > +::
> > +
> > + .------------.  .------------.  .------------.  .------------.
>  .------------.
> > + |            |  |            |  |            |  |            |  |
>        |
> > + | RootBase   <---  Snap-1    <---  Snap-2    <---  Snap-3    <---
>  Snap-4    |
> > + |            |  |            |  |            |  |            |  |
> (Active)   |
> > + '------------'  '------------'  '------------'  '------------'
>  '------------'
> > +                          |                 |              \
> > +                          |                 |               \
> > +                          |                 |                \
> > +                          |                 |                 \ stream
> data
> > +                          |                 | stream data      \
> > +                          | stream data     |                   \
> > +                          |                 |                    v
> > +      .------------.      |                 '--------------->
>  .------------.
> > +      |            |      '--------------------------------->  |
>      |
> > +      | RootBase   |                                           |
>  Snap-4    |
> > +      |            | <---------------------------------------- |
> (Active)   |
> > +      '------------'                 Backing File
>  '------------'
> > +
> > +
> > +
> > +The above figure illustrates that, using block-copy we can pull data
> from
>

.......................................................using blockpull
.................................



> > +Snap-1, Snap-2 and Snap-3 into the 'Active' layer, resulting in
> 'RootBase'
> > +becoming the backing file for the 'Active' image (instead of 'Snap-3',
> which was
> > +the case before doing the blockpull operation).

--------------------------------------------------------------------------------------------------------------

> > +
> > +
> > +Method (2)
> > +----------
> > +To end up with this image chain : **base <- sn1 <- sn3**  (by copying
> *sn2* into *sn3*)
> > +
> > +    (a) Copy contents of sn2(the old backing file) into sn3, and change
> the backing file link of sn3 to sn1::
> > +
> > +        # qemu-img rebase -b sn1.qcow2 sn3.qcow2
> > +
> > +            - Apart from changing backing file link of sn3 to sn1, the
> above cmd
> > +              will it also /copy/ the contents from sn2 into sn3).
> > +
> > +            - In other words: This is 'Safe mode', which is the default
> --
> > +              any clusters that differ between the new  backing_file(in
> this
> > +              case, sn1) and the old backing file(in this case, sn2) of
> > +              filename(in this case, sn3) are merged into
> filename(sn3), before
> > +              actually changing the backing file.
> > +
> > +    (b) Now, we can delete the sn2 disk image(as the changes are now
> committed to
> > +        sn1). ::
>

...........................................................(as the changes
are now merged into sn3).......

> > +
> > +        # rm sn2.qcow2
> > +
> >
>
> This is a good article,I learned a lot from eblake and this article
,thanks all!
 I've translated it into Chinese (http://goo.gl/9xzql) to get more people
join us.
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