[Gllug] Come a cropper with LVM

[David L Neil Mailing list a/c]

Bruce,

Bruce Richardson wrote:
> On Thu, Mar 19, 2009 at 12:48:21PM +0000, David wrote:
>> Bruce Richardson wrote:
>>> On Thu, Mar 19, 2009 at 12:07:36PM +0000, David wrote:
>>>> The disk layout:
>>>> There are three physical Volume Groups (p1, p2, p3). p1 (sda2) and p2
>>>> (sda3) are primary partitions on the same physical drive, contain
>>>> multiple logical volumes and free spaces. p3 is constructed from two
>>>> logical partitions within a single extended partition on a single drive,
>>>> but on physically separate areas of the disk, and consists of a single
>>>> logical volume (LVsrv) and (currently) a little free space.
>>> Erm, this sounds confused to me.  Are p1, p2 and p3 physical volumes or
>>> volume groups?  You don't seem to be clear about the difference.  I'm
>>> guessing that you mean volume groups, looking at the description of p3.
>>> But it doesn't make much sense even if they are volume groups;  why
>>> didn't you just create a bunch of PVs and make one single VG from them?
>>
>> Bruce,
>>
>> Sorry to be confusing:
>> p1, p2, and p3 are individual and separate Volume Groups
>> p1 is a single physical volume
> 
> You mean it contains a single physical volume. 

=correcting my terminology: yes p1 is all of /dev/sda2 and no other, and
all of /dev/sda2 is allocated to LVM's p1.


 A volume group *is not*
> a PV: it consists of one or more pvs and creates a storage pool out of
> all the blocks made available to it by those PVs.

=as applied in p3 (see below)


>> p2 is another single physical volume
>> p3 is made up of two physical volumes
>>
>> I'm not sure that I had a good reason for having multiple VGs instead of
>> just one: questionable logic or a learning experience way-back-when?
> 
> Questionable logic; since these PVs are on the same disk, there's really

=p1 and p2 are one a single disk.

=p3 is on a separate disk and started off as one PV at the 'beginning'
of the drive, and was later extended with another PV further down the
drive-space.


> no gain at all from having different VGs and you suffer a space
> limitation that you would not have if you had unified the PVs into a
> single VG.  If you had multiple PVs on multiple disks, then there can be
> an argument for having one PV from each disk in each VG (but there are
> other ways to enjoy RAID-like performance benefits in LVM).

=no I wasn't going for RAID-like.

=(I note your (and others') later msg indicates that there is no useful
solution - thanks)
it's getting a bit OT but what are the good reasons for creating
multiple VGs cf one large VG (regardless of numbers of disks)?

=Regards,
=dn
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