Re: [linux-lvm] Total free space using added VGs and LVs

[Lou Arnold <larnolda1@gmail.com>]

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Luca, your comments make sense. After my last message I considered just what
you said, but I don't know how to prove it.

I know there is no data on the drive that I added, because I just added the
drive and never put data on it. I am sure "busy" means that it is
mounted. Because it is included in the default group/volume (VolGroup00 -
LogVol00) and because that LV is mounted at root ("/"), I cannot reduce the
filesystem with resize2fs; there is no way to unmount "/", that I know of,
anyway.   Unless of course someone knows how?
On Thu, Oct 22, 2009 at 11:52 PM, Luca Berra <bluca@comedia.it> wrote:

> On Wed, Oct 21, 2009 at 03:03:24PM -0400, Lou Arnold wrote:
>
>> I read the How-To. It doesn't talk about the specific case or being
>> mounted
>>
> I hate how-tos, they are a collection of particular cases and leave the
> luser with a feeling of knowledge. which is not.
>
>
> at root, so I had to experiment. It is likely that commands were in the
>> wrong order, but I don't know what the right order is.
>> I have attached the terminal session I used. In the end it did not work.
>>
>
> the commands were not in the wrong order,
> they were just the wrong commands, unless your aim was reinstalling.
>
>
> There was still 66 GB free, and when I rebooted, the file system failed.
>> The
>> superblock was too big.
>>
>> I obviously don't understand the difference between pvresize, lvreduce and
>> vgreduce, and how  resize2fs  is related to these commands.
>>
>
> I think you need to go over the basics again
> LVM is used to abstract storage management
> it is done by creating layers
> Physical Volumes: which represent disks (or partitions, or whatever
> block device...)
> Volume Group: which is a collection of disks
> Logical Volume: which is a portion of a volume group
>
> LVM allows to add/remove PVs to/from a VG. Add/remove/increase/shrink
> LVs in a VG.
> This is done by dividing each PV in Physical Extents (PE), and then
> mapping those to Logical Extenst (LE) in a LV, so a LV is composed of
> chunks of disk taken from one or more PV in a VG.
>
> When using lvm you create filesystems over logical volumes instead of
> creating them on disk (or partition....)
>
> Lvm has no knowledge of what lays over it, a logical volume
> is just a block device.
>
> The above sentence means that if you use a logical volume to host a
> filesystem and want to resize the lv, you have to deal with the
> filesystem yourself.
> i.e.
> if you enlarge a LV, you have to tell the filesystem that the space
> available has increased.
> if you want to reduce an LV, you have to ensure _before_ doing it that
> the space removed does not contain any data.
> so if you want to reduce an LV containing a filesystem you _have_ to
> tell the filesystem _before_ to let that space alone. if you fail to do
> this you will loose all data that was on the portion of disk you
> removed, and the filesystem will still think it can use that portion of
> data, until it will actually try, then sudden realization will hit like
> a brick. as you just discovered.
>
> btw, let pvresize alone, it is used only in the particular case in which
> you are able to resize the disk underlying a volume group, which is
> impossible for a plain disk.
>
> L.
>
>
>
>
>
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>

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