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Re: parted/LVM for ET [Re: [Libvir] Storage manager initial requirements and thoughts

Richard W.M. Jones wrote:
Richard W.M. Jones wrote:
lvcreate -L 3G -n newroot raidvg
lvcreate -L 1G -n newswap raidvg

I should add, in a libvirt context it's probably going to be useful to also:

* list available volume groups (vgscan)
* list space available in each VG (vgdisplay name-of-vg)
* show how VGs relate to PGs (pvscan)

The LVM tools use quite a sophisticated internal API which isn't exported - but see for example tools/tools.h and include/*.h in the LVM2 source code. <libdevmapper.h> is also available, but much more lowlevel.

I talked to Alasdair Kergon about all of this on Tuesday, and there are a few clarifications.

Firstly, vgscan & vgdisplay are apparently deprecated (who knew?) and there is a new command (vgs) which replaces both. The good news is that vgs is designed so that the output can be parsed:

# /usr/sbin/vgs
  VG         #PV #LV #SN Attr   VSize   VFree
  Home         1   1   0 wz--n- 298.09G  8.09G
  VolGroup00   1   2   0 wz--n- 152.56G 32.00M

versus the old way:

# /usr/sbin/vgdisplay Home
  --- Volume group ---
  VG Name               Home
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  2

Similarly, pvs instead of pvscan/pvdisplay and lvs instead of lvscan/lvdisplay.

Secondly there is an API of sorts for lvm2. I think Alasdair called it "libcmd", but maybe I got that wrong because Google doesn't seem to turn up anything. In any case, all it is is a wrapper around the command line tools, so it seems doubtful that this is going to be any better than just invoking the command line tools ourselves.

Thirdly, Alasdair is quite keen to see a proper programming API for LVM2. However I don't think anyone has the time to do it, and he has the usual concerns for backwards binary compatibility and making sure the API mirrors the command line tools. LVM2 is still evolving at a fair rate. There are few other candidates to use the API at the moment, but apparently Anaconda does some really nasty stuff directly on LVM2 structures so it should be changed to use an API instead.

My current opinion is that we can use libparted as it is to both manage virt-on-partition scenarios and to find orphan PVs as candidates to add to a VG, and the command line tools like vgs to manage and allocate LVM2 partitions.


Emerging Technologies, Red Hat  http://et.redhat.com/~rjones/
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 and make dark of the things which are in their nature excessively
 obvious and simple" (Francis Maseres FRS, mathematician, 1759)

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