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#1 2015-12-02 23:30:45

jinal
Member
Registered: 2014-10-26
Posts: 19

[Abandoned]Free space left of LVM PV partition, extend PV to left

I have a 2TB hdd, which originally contained one big ntfs partition containing all the data. I wish to eventually use the drive completely as one big lvm container, because of the flexibility lvm provides.

I started by backing up some of the data externally and shrinking the ntfs partition, keeping the same start sector. Then I created an lvm PV, VG and a bunch of LVs and copied the backup back into the new logical volumes. It all works fine. Now I've managed to shrink the ntfs partition even further and am trying to extend the PV to the left without breaking anything. Here's the latest fdisk output:


Disk /dev/sdc: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x6ae365c5

Device     Boot      Start        End    Sectors   Size Id Type
/dev/sdc1           411648 1812703231 1812291584 864.2G  7 HPFS/NTFS/exFAT
/dev/sdc2  *    3821486080 3898816511   77330432  36.9G 83 Linux
/dev/sdc4       2152366080 3821486079 1669120000 795.9G 8e Linux LVM

A small side note, in attempting to do this, I tried the following:
Using fdisk, I deleted the sdc4, the partition which was the VG. Then created a new partition with start 1812291585; right next to (now smaller) ntfs partition. When I rebooted, pvdisplay showed no volumes; could not do pvresize. I am not sure if I had set the new partition type correctly to lvm (8e). Anyway, to get my data back, I reverted the old partition definition; after reboot everything seems to work again.

So the question is, how can I expand the PV to the left, with least amount of data transfer. From how I understand lvm works, it should be doable with no data transfer, meta data excluded.

I'd rather not create a new PV; it has a MBR table, I am limited to 4 primary partitions. I cannot continue this strategy until I eventually put all the data into lvm.

Here's some more information


sudo pvdisplay
  --- Physical volume ---
  PV Name               /dev/sdc4
  VG Name               MainVolGroup
  PV Size               795.90 GiB / not usable 3.00 MiB
  Allocatable           yes (but full)
  PE Size               4.00 MiB
  Total PE              203749
  Free PE               0
  Allocated PE          203749
  PV UUID               zOVoH6-ulaa-DB70-EmOP-4UgV-k3Es-ZK3r18
sudo vgdisplay
  --- Volume group ---
  VG Name               MainVolGroup
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  9
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                4
  Open LV               0
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               795.89 GiB
  PE Size               4.00 MiB
  Total PE              203749
  Alloc PE / Size       203749 / 795.89 GiB
  Free  PE / Size       0 / 0   
  VG UUID               NUSnNO-bOOO-xFqi-KyPK-tLHQ-vSuF-B1P6YB
sudo lvdisplay
  --- Logical volume ---
  LV Path                /dev/MainVolGroup/Photos
  LV Name                Photos
  VG Name                MainVolGroup
  LV UUID                93goGD-ebd0-Q4KZ-Xp3i-FJjn-C5rp-2NanV6
  LV Write Access        read/write
  LV Creation host, time jArch, 2015-12-02 19:03:35 +0530
  LV Status              available
  # open                 0
  LV Size                300.00 GiB
  Current LE             76800
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:0
   
  --- Logical volume ---
  LV Path                /dev/MainVolGroup/LinuxPackages
  LV Name                LinuxPackages
  VG Name                MainVolGroup
  LV UUID                tdNx2A-HYAd-JpSN-R4xt-NHMY-kb79-232Wmc
  LV Write Access        read/write
  LV Creation host, time jArch, 2015-12-02 19:31:37 +0530
  LV Status              available
  # open                 0
  LV Size                50.00 GiB
  Current LE             12800
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1
   
  --- Logical volume ---
  LV Path                /dev/MainVolGroup/Archive
  LV Name                Archive
  VG Name                MainVolGroup
  LV UUID                yXArxd-EeMp-NdPc-ktU5-B9db-7kz9-HA5iBy
  LV Write Access        read/write
  LV Creation host, time jArch, 2015-12-02 20:36:40 +0530
  LV Status              available
  # open                 0
  LV Size                300.00 GiB
  Current LE             76800
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:2
   
  --- Logical volume ---
  LV Path                /dev/MainVolGroup/Multimedia2
  LV Name                Multimedia2
  VG Name                MainVolGroup
  LV UUID                I19s4O-Buc3-K75h-zkLp-Ouds-Es99-Of22r1
  LV Write Access        read/write
  LV Creation host, time jArch, 2015-12-02 22:30:04 +0530
  LV Status              available
  # open                 0
  LV Size                145.89 GiB
  Current LE             37349
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:3

Last edited by jinal (2015-12-10 21:01:04)

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#2 2015-12-02 23:37:43

frostschutz
Member
Registered: 2013-11-15
Posts: 1,418

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

Someone asked the same thing yesterday on IRC.

I don't know of a tool that extends PV to the left. It can be done manually be editing LVM metadata, but it has pitfalls - you have to extend left by exact multiple of PE-Size OR adapt the 1st PE Offset accordingly so that your existing PE will still have the correct alignment. LVM allocates space in chunks of 4MiB, if your partition starts at the wrong offset each PE will be 3MiB of old PE + 1 MiB of following old PE and the data you see is garbage.

The easy obvious way is to create a new partition in the free space and just add yet another PV to your VG.

If that's not possible because of msdos partitions, it may be possible to losslessly convert from msdos to gpt partitioning scheme.

If that's not possible either (because Windows or whatever) and you're not confident about your metadata editing skills [do it wrong and all data gone], you're left with moving all data in the partition itself.

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#3 2015-12-03 14:07:05

jinal
Member
Registered: 2014-10-26
Posts: 19

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

Thanks, frostschutz for detailed answer. I am feeling adventurous .. I might try messing with lvm metadata. Any additional information you can give in this direction? Even a set of search terms will be a good start.

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#4 2015-12-03 14:36:55

frostschutz
Member
Registered: 2013-11-15
Posts: 1,418

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

If you add 1000 PE (4000 MiB if your PE size is 4 MiB) to the left of the PV, in the metadata all PE references _for that PV only_ have to be increased by 1000.

So if in the metadata a segment says

				stripes = [
					"pv0", 832
				]

You have to change it to 1832 instead, if you added exactly 1000 PE to the left.

Do not confuse this with the start_extent value which references to the logical offset of the LV which remains unchanged, so the first segment of each LV will always have start_extent 0, and if there are other segments they may have start_extent=2048 or what ever. Those do not change.

To be on the safe side you should run such experiments in read only mode on an overlay (google kernel raid wiki overlay) before commiting it to disk. And of course you should always have a backup of your data regardless.

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#5 2015-12-03 15:17:51

jinal
Member
Registered: 2014-10-26
Posts: 19

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

That's great, thanks frostschutz! I'll need time to experiment with this.

Meanwhile, if someone else is in similar situation, here are some links I've found useful so far.
To create an read only overlay(as suggested by frostschutz): https://raid.wiki.kernel.org/index.php/ … erlay_file
Typical LVM Physical Volume Layout : https://www.centos.org/docs/5/html/Clus … lumes.html

Also more information about my system from vgcfgbackup:

# Generated by LVM2 version 2.02.133(2) (2015-10-30): Thu Dec  3 20:06:42 2015

contents = "Text Format Volume Group"
version = 1

description = "vgcfgbackup -f VG_Info MainVolGroup"

creation_host = "jArch"	# Linux jArch 4.2.5-1-ARCH #1 SMP PREEMPT Tue Oct 27 08:13:28 CET 2015 x86_64
creation_time = 1449153402	# Thu Dec  3 20:06:42 2015

MainVolGroup {
	id = "NUSnNO-bOOO-xFqi-KyPK-tLHQ-vSuF-B1P6YB"
	seqno = 9
	format = "lvm2"			# informational
	status = ["RESIZEABLE", "READ", "WRITE"]
	flags = []
	extent_size = 8192		# 4 Megabytes
	max_lv = 0
	max_pv = 0
	metadata_copies = 0

	physical_volumes {

		pv0 {
			id = "zOVoH6-ulaa-DB70-EmOP-4UgV-k3Es-ZK3r18"
			device = "/dev/sdc4"	# Hint only

			status = ["ALLOCATABLE"]
			flags = []
			dev_size = 1669117952	# 795.897 Gigabytes
			pe_start = 2048
			pe_count = 203749	# 795.895 Gigabytes
		}
	}

	logical_volumes {

		Photos {
			id = "93goGD-ebd0-Q4KZ-Xp3i-FJjn-C5rp-2NanV6"
			status = ["READ", "WRITE", "VISIBLE"]
			flags = []
			creation_host = "jArch"
			creation_time = 1449063215	# 2015-12-02 19:03:35 +0530
			segment_count = 1

			segment1 {
				start_extent = 0
				extent_count = 76800	# 300 Gigabytes

				type = "striped"
				stripe_count = 1	# linear

				stripes = [
					"pv0", 0
				]
			}
		}

		LinuxPackages {
			id = "tdNx2A-HYAd-JpSN-R4xt-NHMY-kb79-232Wmc"
			status = ["READ", "WRITE", "VISIBLE"]
			flags = []
			creation_host = "jArch"
			creation_time = 1449064897	# 2015-12-02 19:31:37 +0530
			segment_count = 1

			segment1 {
				start_extent = 0
				extent_count = 12800	# 50 Gigabytes

				type = "striped"
				stripe_count = 1	# linear

				stripes = [
					"pv0", 76800
				]
			}
		}

		Archive {
			id = "yXArxd-EeMp-NdPc-ktU5-B9db-7kz9-HA5iBy"
			status = ["READ", "WRITE", "VISIBLE"]
			flags = []
			creation_host = "jArch"
			creation_time = 1449068800	# 2015-12-02 20:36:40 +0530
			segment_count = 1

			segment1 {
				start_extent = 0
				extent_count = 76800	# 300 Gigabytes

				type = "striped"
				stripe_count = 1	# linear

				stripes = [
					"pv0", 89600
				]
			}
		}

		Multimedia2 {
			id = "I19s4O-Buc3-K75h-zkLp-Ouds-Es99-Of22r1"
			status = ["READ", "WRITE", "VISIBLE"]
			flags = []
			creation_host = "jArch"
			creation_time = 1449075604	# 2015-12-02 22:30:04 +0530
			segment_count = 1

			segment1 {
				start_extent = 0
				extent_count = 37349	# 145.895 Gigabytes

				type = "striped"
				stripe_count = 1	# linear

				stripes = [
					"pv0", 166400
				]
			}
		}
	}
}

I do not yet know how to edit the lvm metadata(or create a modified copy of original in the correct sector). I suspect I would have to use something like 'dd'.
Any additional help is welcome. I'll post if there are any updates.

Last edited by jinal (2015-12-03 17:41:15)

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#6 2015-12-08 18:45:57

jinal
Member
Registered: 2014-10-26
Posts: 19

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

I am no longer pursuing this. Decided to move data out and create a new LVM VG. Marking as closed.

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#7 2015-12-08 19:38:48

jasonwryan
Anarchist
From: .nz
Registered: 2009-05-09
Posts: 30,424
Website

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

Please don't use [Closed] as it denotes a locked thread. Use [Solved] or [Resolved] or [Abandoned] or [Gave Up] instead...

Arch + dwm   •   Mercurial repos  •   Surfraw

Registered Linux User #482438

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#8 2015-12-10 21:02:13

jinal
Member
Registered: 2014-10-26
Posts: 19

Re: [Abandoned]Free space left of LVM PV partition, extend PV to left

Sorry, my bad. Changed it to [Abandoned] . Thanks smile

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