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

[Matthias Leopold]

Am 2017-07-13 17:25, schrieb Zdenek Kabelac:
> 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
>> 
> 
> 
> Hi
> 
> 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.
> 
> Regards
> 
> Zdenek

Hi Zdenek,

thanks for explaining. I practically knew it was impossible when i wrote 
it, but sometimes i get carried away by fixed ideas and need someone 
"authoritative" to tell me the obvious (with maybe a little technical 
background)... you did the job ;-)

Regards

matthias