Re: vg disappeared after replacing disc in raid10

"Björn Nadrowski" <bjrnfrdnnd@gmail.com> · Tue, 26 Mar 2013 19:13:34 +0100

On Mar 26, 2013, at 14:41 , Stuart D Gathman <stuart@bmsi.com> wrote:

Long ago, Nostradamus foresaw that on 03/25/2013 05:45 PM, Björn
Nadrowski would write:
You were right,
the data has been overwritten.
It now reads thus (or rather, the first instance of human-readable
volume group description reads thus; my old volume group description
is still there, but further down...):
Now you are getting somewhere.  The offset of your PV has shifted. 
First, copy that 2nd human readable part to removable media somewhere. 
If possible, backup your drives (e.g. to USB drives) so you can recover
from further mistakes.

Why has the offset shifted?  Possibilities:

1) different md driver version on knoppix (although using an older
version would move the md superblock from end of drive to beginning of
drive - shifting the opposite way).  Or different options.  BUT mdadm
seemed to recognize your RAID, so let's
discount this theory.

2) alignment options to pvcreate - but it still should have been seen by
lvs!

3) the offset hasn't shifted, but you've written over the first few
sectors with the empty VG.  Is the 2nd human readable part, by any
chance, missing the first part?  You could post both parts for us to
look at.

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Thanks for all these hints. 
I can actually just restore the first bytes of the device as I saved the output of the original dd command to a file. 

sudo dd if=/dev/md127 bs=512 count=255 skip=1 of=md0.txt

So I could just do 
sudo dd if=md0.txt bs=512 count=255 skip=1 of=/dev/md127

Might this be the appropriate action?

I doubt that the entire array has shifted, as md0.txt contained two complete human-readable descriptions of my volume groups and my 
logical volumes. This is the human-readable part of md0.txt (result of the dd command right after disc replacement):

------------------------------
vol0 {
id = "2daJft-nq2X-zJen-2VO1-c2Od-HREv-zyjKTz"
seqno = 7
status = ["RESIZEABLE", "READ", "WRITE"]
extent_size = 8192
max_lv = 0
max_pv = 0

physical_volumes {

pv0 {
id = "WFo1On-anFb-2av8-DuRq-vKee-nJEt-ZLnu26"
device = "/dev/md0"

status = ["ALLOCATABLE"]
dev_size = 5820316928
pe_start = 384
pe_count = 710487
}
}

logical_volumes {

root {
id = "iO0y32-L53R-msRI-3JiE-t7j1-Ija2-3SOnpC"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 11920

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 0
]
}
}

home {
id = "1Vk6QQ-c3NF-6z6h-zw7E-fkFU-mRWU-DKLM6t"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 674725

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 11920
]
}
}
}
}
# Generated by LVM2 version 2.02.39 (2008-06-27): Wed Jun 10 13:21:55 2009

contents = "Text Format Volume Group"
version = 1

description = ""

creation_host = "cn-b204-4"	# Linux cn-b204-4 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:58:03 UTC 2009 x86_64
creation_time = 1244640115	# Wed Jun 10 13:21:55 2009

vol0 {
id = "2daJft-nq2X-zJen-2VO1-c2Od-HREv-zyjKTz"
seqno = 8
status = ["RESIZEABLE", "READ", "WRITE"]
extent_size = 8192
max_lv = 0
max_pv = 0

physical_volumes {

pv0 {
id = "WFo1On-anFb-2av8-DuRq-vKee-nJEt-ZLnu26"
device = "/dev/md0"

status = ["ALLOCATABLE"]
dev_size = 5820316928
pe_start = 384
pe_count = 710487
}
}

logical_volumes {

root {
id = "iO0y32-L53R-msRI-3JiE-t7j1-Ija2-3SOnpC"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 11920

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 0
]
}
}

home {
id = "1Vk6QQ-c3NF-6z6h-zw7E-fkFU-mRWU-DKLM6t"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 674725

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 11920
]
}
}

empty {
id = "YToY9o-223w-nMrv-X5qU-0S8t-oUbG-Zk6ZmV"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 23842

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 686645
]
}
}
}
}
# Generated by LVM2 version 2.02.39 (2008-06-27): Wed Jun 10 13:22:04 2009

contents = "Text Format Volume Group"
version = 1

description = ""

creation_host = "cn-b204-4"	# Linux cn-b204-4 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:58:03 UTC 2009 x86_64
creation_time = 1244640124	# Wed Jun 10 13:22:04 2009

---------------------------

This is the human-readable part of the beginning of the device after my vgcreate action:

---------------------------

vol0 {
id = "8kEHPL-yDuF-7ffg-GeGH-dEg6-D0Qn-lUrKoh"
seqno = 1
format = "lvm2" # informational
status = ["RESIZEABLE", "READ", "WRITE"]
flags = []
extent_size = 8192
max_lv = 0
max_pv = 0
metadata_copies = 0

physical_volumes {

pv0 {
id = "WFo1On-anFb-2av8-DuRq-vKee-nJEt-ZLnu26"
device = "/dev/md127"

status = ["ALLOCATABLE"]
flags = []
dev_size = 5820316928
pe_start = 384
pe_count = 710487
}
}

}
# Generated by LVM2 version 2.02.95(2) (2012-03-06): Sun Mar 24 23:39:50 2013

contents = "Text Format Volume Group"
version = 1

description = ""

creation_host = "Microknoppix"	# Linux Microknoppix 3.6.11 #12 SMP PREEMPT Thu Dec 20 04:04:10 CET 2012 i686
creation_time = 1364168390	# Sun Mar 24 23:39:50 2013

vol0 {
id = "2daJft-nq2X-zJen-2VO1-c2Od-HREv-zyjKTz"
seqno = 8
status = ["RESIZEABLE", "READ", "WRITE"]
extent_size = 8192
max_lv = 0
max_pv = 0

physical_volumes {

pv0 {
id = "WFo1On-anFb-2av8-DuRq-vKee-nJEt-ZLnu26"
device = "/dev/md0"

status = ["ALLOCATABLE"]
dev_size = 5820316928
pe_start = 384
pe_count = 710487
}
}

logical_volumes {

root {
id = "iO0y32-L53R-msRI-3JiE-t7j1-Ija2-3SOnpC"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 11920

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 0
]
}
}

home {
id = "1Vk6QQ-c3NF-6z6h-zw7E-fkFU-mRWU-DKLM6t"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 674725

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 11920
]
}
}

empty {
id = "YToY9o-223w-nMrv-X5qU-0S8t-oUbG-Zk6ZmV"
status = ["READ", "WRITE", "VISIBLE"]
segment_count = 1

segment1 {
start_extent = 0
extent_count = 23842

type = "striped"
stripe_count = 1	# linear

stripes = [
"pv0", 686645
]
}
}
}
}
# Generated by LVM2 version 2.02.39 (2008-06-27): Wed Jun 10 13:22:04 2009

contents = "Text Format Volume Group"
version = 1

description = ""

creation_host = "cn-b204-4"	# Linux cn-b204-4 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:58:03 UTC 2009 x86_64
creation_time = 1244640124	# Wed Jun 10 13:22:04 2009

---------------------------------

It seems, in both cases I have the description of the two last configurations of my volume group. The first one is the actual one. 
Both are complete.

So, should I rewrite the beginning of the device with the dd command?

What then?

Thanks a lot for all your help!

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