[linux-lvm] LVM /dev and /proc problems and change proposal

[Heinz J. Mauelshagen]

Hi Dale.

On Sun, Jun 25, 2000 at 06:35:52PM +1200, Dale Kemp wrote:
> Hi all,
> 
> I've started using LVM thanks to Daniel Mehrmann 2.2.16 kernel patch
> and the debian lvm package (apt-get install lvm). I notice that vg's and
> 
> lv's are created in /dev this means the volume name is limited since it
> could
> clash with similar named devices in the /dev directory. For example:
> 
> # ls -l /dev/par0
> crw-rw----    1 root     lp         6,   0 Feb 23  1999 /dev/par0
> 
> # pvscan
> pvscan -- reading all physical volumes (this may take a while...)
> pvscan -- ACTIVE   PV "/dev/hda6" of VG "vdisk1" [3 GB / 3 GB free]
> pvscan -- inactive PV "/dev/hde5"  is in no VG  [762.86 MB]
> pvscan -- inactive PV "/dev/hde6"  is in no VG  [285.93 MB]
> pvscan -- total: 3 [4.02 GB] / in use: 1 [3 GB] / in no VG: 2 [1.02 GB]
> 
> (create a volume group called par0)
> 
> # vgcreate par0 /dev/hde6
> vgcreate -- INFO: using default physical extent size 4 MB
> vgcreate -- INFO: maximum logical volume size is 255.99 Gigabyte
> vgcreate -- doing automatic backup of volume group "par0"
> vgcreate -- volume group "par0" successfully created and activated
> 
> # ls -ld /dev/par0
> dr-xr-xr-x    2 root     root         1024 Jun 25 17:20 /dev/par0
> 
> Ouch!

You are right, this has not to happen.

It has already been fixed in 0.8final but was still possible in the older
prerelease you are using.

> The device has been replace with a directory! Just adding
> protection here doesn't solve the problem of potential name clashes.

But It dissallows the user to do it and there's still plenty of free namespace
to find a meaningfull name for the Volume Group.

> I think the solution is to store these names in /etc/lvm* and use
> another directory level in /dev.
> 
> e.g.. /dev/lvm/
>       |
>       +---- lvm (109,0)
>       |
>       +---- <name1> +---- group (109,0)
>       |             |
>       |             +---- <lv_name1> (58,0)
>       |             +---- <lv_name2> (58,1)
>       |
>       +---- <name2> +---- group (109,1)
>       |             |
>       |             +---- <lv_name3> (58,2)
>       |
>       +---- etc (not needed if add vg_ and lv_ to above names)
>             |
>             +---- future_item (890,0)

There's a preprocessor definition LVM_DIR_PREFIX in the code, which
supports this already.

Not much testing though...

> 
> OR...
> 
>     /dev/lvm
>       |
>       |
>       +-- <name1> (109,0) [optional forced vg_ prefix to stop
> name-clash]
>       +-- <name2> (109,1)
>       |
>       +-- <name1> (58,0) [optional forced lv_ prefix to stop name-clash]
> 
>       +-- <name2> (58,1)
>       +-- <name3> (58,2)
>       |
>       +-- future_item (890,0)
> 
> OR... (my favorite)
> 
>     /dev/lvm
>       |
>       |
>       +-- pv (example to show future expansion options)
>       |    |
>       |    +-- pv0 --> /dev/hda6
>       |    +-- pv1 --> /dev/hde5
>       |    +-- pv2 --> /dev/hde6
>       |
>       +-- vg
>       |    |
>       |    +-- <name1> (109,0)
>       |    +-- <name2> (109,1)
>       |
>       +-- lv
>       |    |
>       |    +-- <name1> (58,0)
>       |    +-- <name2> (58,1)
>       |    +-- <name3> (58,2)
>       |
>       +-- future_item (890,0)

I don't like both, because they break the hierarchy of LVs (like virtual
partitions) contained in VGs (like virtual disks containing physical disks).

> 
> I use the prefix 'vg_' and 'lv_' to stop any future name clashes. I
> don't see
> it being a good idea to list lv under the vg's since you might allow the
> transfer
> of lv's between vg's.

A command for LV transfer support is not planned.

You can achive it today by merging two volume groups, use pvmove to seperate
allocated PEs to PVs you like and split the VG afterwards.

This can be wrapped by a script to have a single command to move the LV.

> We use the tools to find out more info, or create
> readable
> files in /etc/...
> 
> Also I think there should be a sub-directory for lvm itself in /proc
> ie. /proc/lvm/lvm instead of /proc/lvm.

Yep.
Already on the TODO list.

> 
> Example:
> 
>     # pvcreate /dev/hde[56]
>     # vgcreate vdisk2 /dev/hde5 /dev/hde6
>     # lvcreate -n XFilesVol -L 150M vdisk2
>     # mke2fs /dev/lvm/lv/XFilesVol
>     # mount -t ext2 /dev/lvm/lv/XFileVol /mnt/XFiles
> 
> The current system has name space problems and will be more difficult to
> extend in the future.

If you refer to /dev IMO you are wrong because there's plenty of namespace.

I share your point of view in regard to /proc.


-- 

Regards,
Heinz      -- The LVM guy --

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Heinz Mauelshagen                                 Sistina Software Inc.
Senior Consultant/Developer                       Bartningstr. 12
                                                  64289 Darmstadt
                                                  Germany
Mauelshagen at Sistina.com                           +49 6151 7103 86
                                                       FAX 7103 96
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