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

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

[Matthias Leopold]

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

thx a lot
matthias

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

[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

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

[Stuart D. Gathman]

On Thu, 13 Jul 2017, Zdenek Kabelac wrote:

> 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

The AIX system used a single segment boot volume LV so that bootstrap
code needed only an offset and did not need to understand the LVM. 
But the boot volume was a normal LV in all other respects.  I think
there was a flag on the LV to ensure it remained a single segment.

-- 
 	      Stuart D. Gathman <stuart@gathman.org>
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

Re: [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

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

[Marek Podmaka]

Hello Zdenek,

Thursday, July 13, 2017, 17:25:10, you wrote:

> 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.

Wouldn't creating a partition with correct start sector work as a permanent
solution? (Although I didn't understand if using a partition is an
option for the original poster.)

-- 
  bYE, Marki

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

[Stuart Gathman]

On 07/13/2017 09:25 AM, Matthias Leopold wrote:
> 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
I use iSCSI at home, and I just export the LV as an iSCSI target.  Now
you have direct LUN for use with VM hosts on the network (I just have a
gigabit LAN at home).  The iSCSI server still uses LVM, and you can
snapshot the volume for backup on the server.

Maybe that's too obvious, and I misunderstood your problem?

There is no LVM metatdata on the exported iSCSI devices - that is on the
underlying disk.  As the export LUN looks like a disk, you can even add
an optional partition table, PVs, VGs, etc for use by the virtual machine.