[linux-lvm] convert LV to physical device _in_place_?

Zdenek Kabelac zkabelac at redhat.com
Thu Jul 13 15:25:10 UTC 2017

Dne 13.7.2017 v 15:25 Matthias Leopold napsal(a):
> hi,
> i'm fiddling around with LVM backed KVM raw disks, that i want to use 
> _directly_ in oVirt virtualization (as "Direct LUN"). i would like to avoid 
> "importing", dd, etc. if possible. in the KVM origin system exists a mapping 
> of one iSCSI whole device (no partitions) to one PV to one VG to one LV per 
> KVM disk. i can now present these iSCSI devices to the oVirt hosts, where i 
> can only use them as "Direct LUN" without the LVM layer (i guess). so all i 
> would need is to remove the LVM metadata from the iSCSI device preserving the 
> "content" of the LV. pvremove seems to be too "intrusive" for this... i know 
> this sounds rather naive and i didn't find any recipes for it. everybody talks 
> about dd to a new device, which of course works (i tried it), but is exactly 
> what i want to avoid when migrating very large disks (TB). i hope someone 
> understands my concern and maybe has a solution


Yep, it really sounds a bit naive :)

PV has 'header' so the real 'data' are shifted by  PV header+lvm2 metadata.
and also LV does not need to be sequential.

However if you have been having a single 'segment' LV and and you calculate
proper skipping offset (typically 1MB) you can try to use such device directly 
without lvm2 with a loop device mapping -  see losetup --offset option - but 
you would need to use this everytime you want to access such storage - so 
there is not a single magic 'flip' which would make an LV to appear like
regular device.



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