[linux-lvm] If one disk fails i loose everything?

[linux-lvm] If one disk fails i loose everything?

[kyr]

  I have tree simple questions about LVM that just could not be answered
  after 4 hours of search in LVM howto, Google, Linux questions and
  linux-lvm Archives.


  I have a small file server with 600Gbytes in one liner volume group
  consisting of 3 200 Gbyte Hard disk. My questions are:


  1. If one hard disk fails (hardware) do I loose all the data stored on
  the VG?


  2. Can I add a new hard disk in the VG without having to format it
  before? (I mean if it is full of data can I just add it?)


  3. In case of failure can I recover the data from a single disk on
  another box?


  Thanks in advance


  Kyriakos


  Greece

Re: [linux-lvm] If one disk fails i loose everything?

[Erik Ohrnberger]

On Tue, December 6, 2005 18:37, kyr wrote:
>
>   I have tree simple questions about LVM that just could not be answered
>   after 4 hours of search in LVM howto, Google, Linux questions and
>   linux-lvm Archives.
>
>
>   I have a small file server with 600Gbytes in one liner volume group
>   consisting of 3 200 Gbyte Hard disk. My questions are:
>
>
>   1. If one hard disk fails (hardware) do I loose all the data stored on
>   the VG?

    Yup.  That's how it works.  There are programs that can scavange
partial data from the hard disks that make up the VG.  r-studio being
one of them.

>   2. Can I add a new hard disk in the VG without having to format it
>   before? (I mean if it is full of data can I just add it?)

    You'll have to create an LVM data partition for the data that you want
to store on the drive, and this will wipe out the currently existing
data on the drive.

>   3. In case of failure can I recover the data from a single disk on
>   another box?

    Possibly, but I've never done this.  Seems rather doubtful to me.

>   Thanks in advance
>
>
>   Kyriakos
>
>
>   Greece
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>

Re: [linux-lvm] If one disk fails i loose everything?

[gwood]

In the answers below I've taken 'one linear volume group' to actually mean
'one linear volume using all the space in the volume group'.  If this is
not what you meant, can you please describe your setup?

>   1. If one hard disk fails (hardware) do I loose all the data stored on
>   the VG?
I'm not sure where you searched, but the answer to this is a fundamental
characteristic of RAID0 - there is no redundancy.  If you have a concat
(rather than a stripe), then you /might/ be able to salvage /some/ data -
but the chances are pretty slim that you'll get anything sane back out of
it, and you're going to have to do some relatively low level stuff to do
even that I would have thought.

>   2. Can I add a new hard disk in the VG without having to format it
>   before? (I mean if it is full of data can I just add it?)
Not easily using LVM on linux, no.  If it already has partitions on it,
then the layout of the data on the drive is incorrect for LVM.  There may
be tools out there to overlay the required metadata, but the underlying
partitions will have to reduce in size, so this would require the
filesystems to support being shrunk.

>   3. In case of failure can I recover the data from a single disk on
>   another box?
What sort of failure?  Any VG that has enough drives to provide you with a
working volume group/volume can be imported to another machine and read. 
So if you have 3 volumes over the VG, and only one of them doesn't have a
complete set of blocks on it - the other 2 can be salvaged.  In the case
of the arrangement you're talking about, no.

If you need to be able to recover the data, then a simple linear volume
(with no redundancy) is not a good idea.

(In answer to the subject line, 'with the arrangement you're talking
about' - YES')

Re: [linux-lvm] If one disk fails i loose everything?

[Old Fart]

gwood@dragonhold.org wrote:
> In the answers below I've taken 'one linear volume group' to actually mean
> 'one linear volume using all the space in the volume group'.  If this is
> not what you meant, can you please describe your setup?
>
>   
>>   1. If one hard disk fails (hardware) do I loose all the data stored on
>>   the VG?
>>     
> I'm not sure where you searched, but the answer to this is a fundamental
> characteristic of RAID0 - there is no redundancy.  If you have a concat
> (rather than a stripe), then you /might/ be able to salvage /some/ data -
> but the chances are pretty slim that you'll get anything sane back out of
> it, and you're going to have to do some relatively low level stuff to do
> even that I would have thought.
>
>   
>>   2. Can I add a new hard disk in the VG without having to format it
>>   before? (I mean if it is full of data can I just add it?)
>>     
> Not easily using LVM on linux, no.  If it already has partitions on it,
> then the layout of the data on the drive is incorrect for LVM.  There may
> be tools out there to overlay the required metadata, but the underlying
> partitions will have to reduce in size, so this would require the
> filesystems to support being shrunk.
>
>   
>>   3. In case of failure can I recover the data from a single disk on
>>   another box?
>>     
> What sort of failure?  Any VG that has enough drives to provide you with a
> working volume group/volume can be imported to another machine and read. 
> So if you have 3 volumes over the VG, and only one of them doesn't have a
> complete set of blocks on it - the other 2 can be salvaged.  In the case
> of the arrangement you're talking about, no.
>
> If you need to be able to recover the data, then a simple linear volume
> (with no redundancy) is not a good idea.
>
> (In answer to the subject line, 'with the arrangement you're talking
> about' - YES')
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
>   
Good discussion....the need to protect data under various contingencies 
is why I use raid5 sets as the PVs.  You can lose up to two and keep 
your data, hot add, have spares, etc.

-- 
Regards,

Old Fart