[Libguestfs] [PATCH 00/10] virt-resize: add support for resizing MBR logical partitions

[Chen, Hanxiao]

Hi, Rich

> -----Original Message-----
> From: Richard W.M. Jones [mailto:rjones at redhat.com]
> Sent: Wednesday, April 29, 2015 7:04 PM
> To: Chen, Hanxiao/陈 晗霄
> Cc: libguestfs at redhat.com
> Subject: Re: [Libguestfs] [PATCH 00/10] virt-resize: add support for resizing MBR
> logical partitions
> 
> On Wed, Apr 29, 2015 at 09:19:24AM +0000, Chen, Hanxiao wrote:
> > commit 6e989fd709fea37430398cbb7c01c52cd44bd1ac already did this:
> > we had p_mbr_p_type : partition_type in type partition.
> > Like what MBR/EBR did, this patchset use `partition_type`:
> >
> > /dev/sda3:
> > 	partition data: 3 210763776-420478975 (209715200 bytes)
> > 	bootable: false
> > 	partition ID: 0x5
> > 	content: extended partition
> > 	label: (none)
> > 	GUID: (none)
> > 	*partition type: extended*
> > /dev/sda5:
> > 	partition data: 5 211812352-222298111 (10485760 bytes)
> > 	bootable: false
> > 	partition ID: 0x83
> > 	content: unknown data
> > 	label: (none)
> > 	GUID: (none)
> > 	*partition type: logical*
> >
> > As we could only had one extended partition,
> > all logical partitions belong to that extended partition,
> > so current type partition of virt-resize can reflect logical/extended partition
> model.
> 
> The specific problem is that the extended partition (eg /dev/sda3)
> contains the logical partition (/dev/sda5).  Having a flat list
> doesn't express that at all.

We had at most 4 PTE in MBR, in PTE, a partition type is on +04h, 
value 05h or 0fh shows it's an extended partition.
>From extended partition's EBR, we got logical partitions.

sda1-- sda2 -- sda3 -- sda4
                   ^
                   |
                  sda5 -- sda6

I think we do not need to show this specially,
just use the original design:
logical partitions existed in extended partition, an implicit relations.

So a flat list can do this:
We mark each members, then a tree view above could be rebuilt precisely
from plat list.


In this patchset, logical partitions are not filter out, it did:

1) partitions list: sda1 sda2 sda3 sda5
every member of partitions got a member `partition type`

2) calculate surplus

3) then filter out logical partitions list:
logical_partitions: sda5
partitions: sda1 sda2 sda3
do something to it (calculate_target_partitions...)

4) part_add to 'non-logical' partitions and logical partitions,
then concrete them together for the original partition list.

5) copy_partition (ignore extended partitions, for we had logical partition
in partitions list, we copy data from there)

> 
> I agree it is very hard to express this, because extended partitions
> just work so differently from how any sensible partition scheme should
> work (because they are a big hack).  But perhaps:
> 
>  - flat list of primary partitions (as now, the global 'partitions')

members of partitions list whose partition type is 'primary'
> 
>  - one MBR extended partition (as a new global variable)

member of partitions list whose partition type is 'extended'.

> 
>  - flat list of logical partitions

members of partitions list whose partition type is 'logical'.

So I think we do not need 2 list and a global variable,
if we want to know something, just do a List.filter.

> 
> would do it?  It still has the problem that the extended partition is
> a bit like a primary partition, yet isn't in the partitions list.

For the point of PTE's view, it does.
PS: In my patch, it's in partitions list :)

Regards
- Chen

> 
> > > It's very hard to understand what changes are being made.  The commits
> > > are not grouped in a way that makes them easily understandable to me.
> >
> > I'll try to regroup them and make the patchset easy to review.
> 
> Thanks,
> 
> Rich.
> 
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
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