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#1 2014-01-30 20:17:25

alphazo
Member
Registered: 2009-10-20
Posts: 155

"DM_snapshot_cow signature detected" message

My system has running fine since 2012 (and still does) based upon a LVM space on top of an encrypted LUKS partition that I use for both root and home.
I have some free space on my LVM array that I use to perform system home & root snapshots prior to any Arch upgrade (just in case) that I remove after rebooting correctly.

Since the latest update I did a couple of days I keep getting the following message when creating the snapshot. A friend of mine with the exact same setup also gets the message. We haven't been so adventurous yet to answer "Yes" to the question regarding the wipe operation. What do you think?

#sudo lvcreate -L 4g -s -n root-snapshot /dev/vgroup/root
WARNING: DM_snapshot_cow signature detected on /dev/vgroup/root-snapshot at offset 0. Wipe it? [y/n]  [n]

  1 existing signature left on the device.
  Logical volume "root-snapshot" created

Looks like snapshot creation went fine. Later I can remove it without any issue:

# sudo lvremove /dev/vgroup/root-snapshot
Do you really want to remove active logical volume root-snapshot? [y/n]: y 
  Logical volume "root-snapshot" successfully removed

Here is my LVM setup:

# sudo lvdisplay

  --- Logical volume ---
  LV Path                /dev/vgroup/root
  LV Name                root
  VG Name                vgroup
  LV UUID                9atWzf-Z9ii-ssiW-P51P-4M7o-uGI6-tgS93q
  LV Write Access        read/write
  LV Creation host, time archiso, 2012-11-18 00:29:14 +0100
  LV Status              available
  # open                 1
  LV Size                25,00 GiB
  Current LE             6400
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1
   
  --- Logical volume ---
  LV Path                /dev/vgroup/home
  LV Name                home
  VG Name                vgroup
  LV UUID                tx4ZgI-a2pg-ksVU-vllz-SnWU-VL3p-z0EMSm
  LV Write Access        read/write
  LV Creation host, time archiso, 2012-11-18 00:30:45 +0100
  LV Status              available
  # open                 1
  LV Size                415,00 GiB
  Current LE             106240
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:2

# sudo vgdisplay

  --- Volume group ---
  VG Name               vgroup
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  914
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               445,12 GiB
  PE Size               4,00 MiB
  Total PE              113951
  Alloc PE / Size       112640 / 440,00 GiB
  Free  PE / Size       1311 / 5,12 GiB
  VG UUID               Ragce3-tU7G-jcbz-JIdN-grad-yWFH-VyI4uk

# sudo pvdisplay

  --- Physical volume ---
  PV Name               /dev/mapper/vgroup
  VG Name               vgroup
  PV Size               445,13 GiB / not usable 5,82 MiB
  Allocatable           yes 
  PE Size               4,00 MiB
  Total PE              113951
  Free PE               1311
  Allocated PE          112640
  PV UUID               5cB7tp-yjNe-eskX-gVct-t3YJ-pd1r-JdaL2p

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#2 2014-02-01 10:58:14

ritze
Member
Registered: 2012-11-10
Posts: 6

Re: "DM_snapshot_cow signature detected" message

Same problem on all my machines with lvm.

Here the setup from one of my arch systems:

# sudo lvdisplay:

  --- Logical volume ---
  LV Path                /dev/system/arch
  LV Name                arch
  VG Name                system
  LV UUID                XXXXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXXXX
  LV Write Access        read/write
  LV Creation host, time , 
  LV Status              available
  # open                 1
  LV Size                30,00 GiB
  Current LE             7680
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:0
   
  --- Logical volume ---
  LV Path                /dev/system/home
  LV Name                home
  VG Name                system
  LV UUID                XXXXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXXXX
  LV Write Access        read/write
  LV Creation host, time , 
  LV Status              available
  # open                 1
  LV Size                24,88 GiB
  Current LE             6370
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1

# sudo vgdisplay

  --- Volume group ---
  VG Name               system
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3618
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               59,12 GiB
  PE Size               4,00 MiB
  Total PE              15135
  Alloc PE / Size       14050 / 54,88 GiB
  Free  PE / Size       1085 / 4,24 GiB
  VG UUID               XXXXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXXXX

# sudo pvdisplay

  --- Physical volume ---
  PV Name               /dev/sda2
  VG Name               system
  PV Size               59,13 GiB / not usable 4,32 MiB
  Allocatable           yes 
  PE Size               4,00 MiB
  Total PE              15135
  Free PE               1085
  Allocated PE          14050
  PV UUID               XXXXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXXXX

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#3 2014-02-02 21:33:23

alphazo
Member
Registered: 2009-10-20
Posts: 155

Re: "DM_snapshot_cow signature detected" message

Thanks for the feedback. So apparently this is only related to LVM since you are apparently not using LUKS encryption. Now were you brave enough to answer Yes to the wipe question?

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#4 2014-02-03 07:22:17

ritze
Member
Registered: 2012-11-10
Posts: 6

Re: "DM_snapshot_cow signature detected" message

alphazo wrote:

Now were you brave enough to answer Yes to the wipe question?

I did this already. Nothing happen. This question appears again on the next creation of a snapshot. (My daily backup remembers me on this. hmm)

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#5 2014-02-08 04:49:31

mistofvongola
Member
Registered: 2013-05-16
Posts: 2

Re: "DM_snapshot_cow signature detected" message

I've also started getting this in my backup scripts recently (uses LVM snapshots). Same as alphazo, LVMs on a LUKS encrypted partition. I didn't see a bug report yet.

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#6 2014-02-08 14:29:25

eworm
Trusted User (TU)
From: Oberhausen, Germany
Registered: 2010-01-30
Posts: 100
Website

Re: "DM_snapshot_cow signature detected" message

I have a different (maybe related bug) and found a solution (or workaround) for yours. The prblem is the blkid wiping code. The message goes  away if you compile the package with --disable-blkid_wiping.


ArchLinux - make it simple & lightweight

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#7 2014-02-09 10:29:20

ritze
Member
Registered: 2012-11-10
Posts: 6

Re: "DM_snapshot_cow signature detected" message

eworm wrote:

I have a different (maybe related bug) and found a solution (or workaround) for yours. The prblem is the blkid wiping code. The message goes  away if you compile the package with --disable-blkid_wiping.

It works! Thank you.

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