Volumes vs. Partitions - I am confused

Hallo,

I am a complete newbie with LVM. Here are my questions:

1.) Is it really true that I can give names to
	a.) disks
	b.) volumes
	c.) partitions

2.) So, what is the meaning/difference between a "disk", a
"volume" and 
a "partition" (yet another abstraction ...) ?

3.) If so, what is the relationship between a volume and a partition ? 
As far as I understand the help, there is a 1:1 relationship between 
volumes and partitions, so why the distinction ?

4.) Do I have to create a volume first before I can create a partition ? 
If not will creation of a partition lead to creation of a volume ?

5.) Why is it that if I create a volume on an empty disk (no matter if 
compatibility or LVM) that always the first 7 MB are lost ? What is in 
those first 7 MB ?

Any help greatly appreciated. I am really worried that I will mess up my 
system if I don't get a clear picture.

Lars Erdmann wrote:
> Hallo,
> 
> I am a complete newbie with LVM. Here are my questions:
> 
> 1.) Is it really true that I can give names to
>     a.) disks
>     b.) volumes
>     c.) partitions
> 
> 2.) So, what is the meaning/difference between a "disk", a
"volume" and 
> a "partition" (yet another abstraction ...) ?

Disk - I think it is obvious
Partition - it is a part of disk, as you probably now it from before
Volume - contains a partition or multiple partitions, thats why the 
volumes are being created.

Nowadays if you want to operate with a part of disk it usualy have to be 
a volume. Partition and disk is more hardware level, volume is more 
software level.

> 
> 3.) If so, what is the relationship between a volume and a partition ? 
> As far as I understand the help, there is a 1:1 relationship between 
> volumes and partitions, so why the distinction ?
you can have two partitions in one volume, thats why the distinction
> 
> 4.) Do I have to create a volume first before I can create a partition ? 
> If not will creation of a partition lead to creation of a volume ?
no it is the other way around, in LVM when you are creating a volume you 
avtomaticly create or/and use a partition.
> 
> 5.) Why is it that if I create a volume on an empty disk (no matter if 
> compatibility or LVM) that always the first 7 MB are lost ? What is in 
> those first 7 MB ?
> 
This is probably because you are creating a extended partition, if you 
are creating a primary partition that is not the case.
Bottomline on a physical disk you "need" at least one primary
partition 
or you will "loose" 7MB

> Any help greatly appreciated. I am really worried that I will mess up my 
> system if I don't get a clear picture.
> 
> 
> Lars

I hope this helps

Marko Udvanc schrieb:
> Lars Erdmann wrote:
>> Hallo,
>>
>> I am a complete newbie with LVM. Here are my questions:
>>
>> 1.) Is it really true that I can give names to
>>     a.) disks
>>     b.) volumes
>>     c.) partitions
>>
>> 2.) So, what is the meaning/difference between a "disk", a
"volume" 
>> and a "partition" (yet another abstraction ...) ?
> 
> Disk - I think it is obvious
> Partition - it is a part of disk, as you probably now it from before
> Volume - contains a partition or multiple partitions, thats why the 
> volumes are being created.
> 
> Nowadays if you want to operate with a part of disk it usualy have to be 
> a volume. Partition and disk is more hardware level, volume is more 
> software level.
> 
>>
>> 3.) If so, what is the relationship between a volume and a partition ?

>> As far as I understand the help, there is a 1:1 relationship between 
>> volumes and partitions, so why the distinction ?
> you can have two partitions in one volume, thats why the distinction
>>

That's only possible for LVM volumes (and not compatibility volumes), 
correct ?
When I create a compatibility volume, it seems to be only possible to 
have exactly 1 partition in it ...

>> 4.) Do I have to create a volume first before I can create a partition

>> ? If not will creation of a partition lead to creation of a volume ?
> no it is the other way around, in LVM when you are creating a volume you 
> avtomaticly create or/and use a partition.
>>
>> 5.) Why is it that if I create a volume on an empty disk (no matter if

>> compatibility or LVM) that always the first 7 MB are lost ? What is in

>> those first 7 MB ?
>>
> This is probably because you are creating a extended partition, if you 
> are creating a primary partition that is not the case.
> Bottomline on a physical disk you "need" at least one primary
partition 
> or you will "loose" 7MB

Ah right.

> 
>> Any help greatly appreciated. I am really worried that I will mess up 
>> my system if I don't get a clear picture.
>>
>>
>> Lars

Thank you very much for your help !

On 09.09.07 21:39, Lars Erdmann wrote:

> Marko Udvanc schrieb:
>
>> you can have two partitions in one volume, thats why the
>> distinction
>
> That's only possible for LVM volumes (and not compatibility
> volumes), correct ?

Yes. Note that you also have to select "LVM volume" when you want to
format it with JFS. And: the partition-spanning feature is only
available for JFS-formatted partitions (or maybe a set of them already
joined to a volume?).

-- 

On Sun, 9 Sep 2007 08:49:14 UTC, Lars Erdmann <lars.erdmann@arcor.de>
wrote:

> I am a complete newbie with LVM. Here are my questions:
> 
> 1.) Is it really true that I can give names to
> 	a.) disks
> 	b.) volumes
> 	c.) partitions

Yes.  Up to 20 characters each (including spaces & punctuation).

 
> 2.) So, what is the meaning/difference between a "disk", a
"volume" and 
> a "partition" (yet another abstraction ...) ?

A disk is a physical, partitionable disk.

A partition is a primary or logical partition.

Volumes are the new concept in the mix.  A volume is basically just a 
small set of metadata which is associated with one or more partitions.

 
> 3.) If so, what is the relationship between a volume and a partition ? 
> As far as I understand the help, there is a 1:1 relationship between 
> volumes and partitions, so why the distinction ?

No, not quite.  A volume is a set of metadata _associated_ with either
a single partition (compatibility volume) or a group of partitions
(LVM volume).

This metadata is basically just a name and (optionally) a drive letter.
The conceptual change here is that OS/2 no longer assigns drive letters
to partitions; it only assigns them to volumes, and only the specific
drive letters that you have chosen.


> 4.) Do I have to create a volume first before I can create a partition ? 
> If not will creation of a partition lead to creation of a volume ?

No to both.  

Think of a volume as an optional "fingerprint" which can be placed 
on a partition.

A partition _can_ be fingerprinted as a volume.  It doesn't have to 
be. 

A partition which is not a volume won't have a drive letter (or be
able to have one), that's all.  In fact, it's quite useful to leave
"foreign" operating system partitions like this, so OS/2 doesn't
have to worry about them at all.  (The LVM front-end actually won't
let you add such partitions to Boot Manager either, but the LVM engine
itself allows it; there are a couple of other programs that will do
this if you want to.)


> 5.) Why is it that if I create a volume on an empty disk (no matter if 
> compatibility or LVM) that always the first 7 MB are lost ? What is in 
> those first 7 MB ?

That has nothing to do with volumes, and everything to do with 
partitions.  If you don't create a primary partition, you lose the
use of the first cylinder on the disk.  So a disk with only logical
partitions with have that 7 MB gap at the start.


> Any help greatly appreciated. I am really worried that I will mess up my 
> system if I don't get a clear picture.

Just think of your disk in the same way you always did.  You have 
one or more disk drives, each of which is divided into partitions.
That's the physical layer, which is no different than it always was.

Volumes are simply a logical abstraction pasted on top of that layer.
Volumes are specific to OS/2: other operating systems neither see 
nor care about them, and they don't affect the physical organization
of your disks or partitions in any way.  As I said, they are purely
metadata - a logical fingerprint that OS/2 LVM places on partitions
in order to associate extra information with them PURELY FOR OS/2's
OWN USE.

When a partition has this metadata added, it becomes a volume in
the new terminology.  (Or, in the case of LVM volumes, it becomes
_part_ of a volume.)

There are really only two things you need to watch out for.  First,
while I said a volume is just metadata, LVM actually uses the term
"volume" to refer to partition(s) PLUS metadata.  So if you choose
to delete a volume, you are actually asking for the partition(s) to
be deleted as well.  Just remember that.

Second, if you use non-LVM-aware software to make modifications to
any specific partitions which are marked as volumes, that metadata
will probably be lost, and you will have to go back into LVM and 
re-designate the partition(s) as volumes again.  (Partition Magic
is the number one suspect here.)


Once you get your head around the concepts, LVM really does simplify
the management of partitions tremendously, because you're no longer
at the mercy of the bizarre logic by which partitions used to be
assigned drive letters automatically (whether you wanted them to be 
or not).

-- 
Alex Taylor
http://www.cs-club.org/~alex