[linux-lvm] Underlying physical volume resized?!

[linux-lvm] Underlying physical volume resized?!

[Gionatan Danti]

Dear all,
I have a question about a warning message LVM is showing. I am using 
CentOS 6.8 x86_64. Here is my pvs/vgs/lvs configuration:

pvs:
   PV         VG   Fmt  Attr PSize   PFree
   /dev/sda2  vg   lvm2 a--u   9,51g    0
   /dev/sda3  vg   lvm2 a--u 110,00g    0
   /dev/sdb1  vg   lvm2 a--u 120,00g 4,00g

vgs:
   VG   #PV #LV #SN Attr   VSize   VFree
   vg     3   2   0 wz--n- 239,50g 4,00g

lvs:
   LV      VG   Attr       LSize   Pool Origin Data%  Meta%  Move Log 
Cpy%Sync Convert
   lv_root vg   -wi-ao---- 233,56g
   lv_swap vg   -wi-ao----   1,94g

Both at kernel discovery (during boot) and when issuing lvs or other LVM 
commands I have the warning:

"Device /dev/sda2 has size of 19945392 sectors which is smaller than 
corresponding PV size of 19945472 sectors. Was device resized? One or 
more devices used as PVs in VG vg have changed sizes."

However, my partitions where never shrunk. fdisk on /dev/sda:

Disk /dev/sda: 128.8 GB, 128849018880 bytes
255 heads, 63 sectors/track, 15665 cylinders, total 251658240 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000a3c73

    Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048     1026047      512000   83  Linux
/dev/sda2         1026048    20971439     9972696   8e  Linux LVM
/dev/sda3        20971440   251658239   115343400   8e  Linux LVM

By fdisk output, lvm is right: 20971439-1026048+1=19945392, so sda2 is 
80 sectors (40 KB) smaller than lvm expects (19945472 sectors). I 
expanded the root volume quite a few times; however, I *never* resized 
sda2: at each expansion I took a snapshot of the disk's MBR, so I 
already checked that I did not mess with sda2 in the past.

By using lvm metadata archived in /etc/lvm/archive, I think that the 
missing 80 sectors are squarely in the swap space (lv_swap, which used 
the last physical extents when lv_root was much smaller), but I am 
somewhat worried that, given some process write to these "missing" 80 
sectors, something bad can happen to the next adjacent physical extent 
(where live data are store, as it is part of lv_root now).

In short:

1) can someone point me on what happen here, and why lvs only recently 
started to complain?

2) do you think my data are at risk?

3) what I can do to solve the problem? I can think of two different 
approach: a) run pvreside /dev/sda2 to shrunk the physical volume or b) 
shrunk the swap partition to be sure nobody will ever write to the last 
80 sectors.

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

Re: [linux-lvm] Underlying physical volume resized?!

[Marian Csontos]

On 11/15/2016 03:27 PM, Gionatan Danti wrote:
> Dear all,
> I have a question about a warning message LVM is showing. I am using
> CentOS 6.8 x86_64. Here is my pvs/vgs/lvs configuration:
>
> pvs:
>   PV         VG   Fmt  Attr PSize   PFree
>   /dev/sda2  vg   lvm2 a--u   9,51g    0
>   /dev/sda3  vg   lvm2 a--u 110,00g    0
>   /dev/sdb1  vg   lvm2 a--u 120,00g 4,00g
>
> vgs:
>   VG   #PV #LV #SN Attr   VSize   VFree
>   vg     3   2   0 wz--n- 239,50g 4,00g
>
> lvs:
>   LV      VG   Attr       LSize   Pool Origin Data%  Meta%  Move Log
> Cpy%Sync Convert
>   lv_root vg   -wi-ao---- 233,56g
>   lv_swap vg   -wi-ao----   1,94g
>
> Both at kernel discovery (during boot) and when issuing lvs or other LVM
> commands I have the warning:
>
> "Device /dev/sda2 has size of 19945392 sectors which is smaller than
> corresponding PV size of 19945472 sectors. Was device resized? One or
> more devices used as PVs in VG vg have changed sizes."

Hi, the warning was added only recently - commit c0912af3 added 2016-01-22:

https://git.fedorahosted.org/cgit/lvm2.git/commit/?id=c0912af3104cb72ea275d90b8b1d68a25a9ca48a

Were the partitions created by anaconda?
This might be an installer bug.

LVM allows overriding PV size to be larger than device size.

More about the feature here: 
https://bugzilla.redhat.com/show_bug.cgi?id=1323950

I do not think pvmove could help here, as there will be read errors on 
the device.

Deactivating swap (`swapoff /dev/$VG/$LV`)
deleting the swap LV (`lvremove $VG/$LV`)
shrinking the PV (simply running `pvresize /dev/sda2` without arguments),
recreating the swap LV (`lvcreate -n $LV -L $SIZE $VG`),
`mkswap /dev/$VG/$LV`
and `swapon /dev/$VG/$LV`
should do.

-- Marian

>
> However, my partitions where never shrunk. fdisk on /dev/sda:
>
> Disk /dev/sda: 128.8 GB, 128849018880 bytes
> 255 heads, 63 sectors/track, 15665 cylinders, total 251658240 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> Disk identifier: 0x000a3c73
>
>    Device Boot      Start         End      Blocks   Id  System
> /dev/sda1   *        2048     1026047      512000   83  Linux
> /dev/sda2         1026048    20971439     9972696   8e  Linux LVM
> /dev/sda3        20971440   251658239   115343400   8e  Linux LVM
>
> By fdisk output, lvm is right: 20971439-1026048+1=19945392, so sda2 is
> 80 sectors (40 KB) smaller than lvm expects (19945472 sectors). I
> expanded the root volume quite a few times; however, I *never* resized
> sda2: at each expansion I took a snapshot of the disk's MBR, so I
> already checked that I did not mess with sda2 in the past.
>
> By using lvm metadata archived in /etc/lvm/archive, I think that the
> missing 80 sectors are squarely in the swap space (lv_swap, which used
> the last physical extents when lv_root was much smaller), but I am
> somewhat worried that, given some process write to these "missing" 80
> sectors, something bad can happen to the next adjacent physical extent
> (where live data are store, as it is part of lv_root now).
>
> In short:
>
> 1) can someone point me on what happen here, and why lvs only recently
> started to complain?
>
> 2) do you think my data are at risk?
>
> 3) what I can do to solve the problem? I can think of two different
> approach: a) run pvreside /dev/sda2 to shrunk the physical volume or b)
> shrunk the swap partition to be sure nobody will ever write to the last
> 80 sectors.
>
> Thanks.
>

Re: [linux-lvm] Underlying physical volume resized?!

[Gionatan Danti]

 > On 18/11/2016 04:43, Marian Csontos wrote:
>
> Hi, the warning was added only recently - commit c0912af3 added 2016-01-22:
>
> https://git.fedorahosted.org/cgit/lvm2.git/commit/?id=c0912af3104cb72ea275d90b8b1d68a25a9ca48a

Ok, this explain why I only recently saw this warning

> Were the partitions created by anaconda?
> This might be an installer bug.
>
> LVM allows overriding PV size to be larger than device size.
>
> More about the feature here:
> https://bugzilla.redhat.com/show_bug.cgi?id=1323950
>
> I do not think pvmove could help here, as there will be read errors on
> the device.

This a cloud VM not directly installed by me, rather from the cloud 
provider. I think they somewhat messed up their template partition table 
but, with no warning and no apparent error, the problem was unrecognized

> Deactivating swap (`swapoff /dev/$VG/$LV`)
> deleting the swap LV (`lvremove $VG/$LV`)
> shrinking the PV (simply running `pvresize /dev/sda2` without arguments),
> recreating the swap LV (`lvcreate -n $LV -L $SIZE $VG`),
> `mkswap /dev/$VG/$LV`
> and `swapon /dev/$VG/$LV`
> should do.

This is a production machine, so I am somewhat reluctant to shrink 
swap/physical volumes. From what I understand (and from my own testing 
on a test machine) this slight discrepancy between partition and PV 
*should* pose no problems to my setup. At most, when swap is full, I can 
receive an error about a failed page swapping.  Is this assumption correct?

Thank you very much.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8