Re: rules for EVMS volumes

["Alexander E. Patrakov" <patrakov@ums.usu.ru>]

Hai Zaar wrote:
> Hi!
> 
> I've wanted EVMS volumes to show in /dev/disk. So I've created the
> following rules and placed them in /etc/udev/rules.d/61-evms.rules
> (right after persistent rules).
> 
> KERNEL="dm-[0-9]*",    ACTION="add", IMPORT{program}="/bin/bash -c
> 'echo DMNAME=$$(/sbin/dmsetup info -c --noopencount --noheadings -o
> name -j %M -m %m)'"
> KERNEL="dm-[0-9]*",    ACTION="add", IMPORT{program}="/sbin/vol_id
> --export $tempnode"
> KERNEL="dm-[0-9]*",    ACTION="add",
> ENV{ID_FS_UUID}="?*",PROGRAM="/bin/test -e /dev/evms/$env{DMNAME}",  
>     SYMLINK+="disk/by-uuid/$env{ID_FS_UUID}"
> KERNEL="dm-[0-9]*",    ACTION="add",
> ENV{ID_FS_LABEL_SAFE}="?*",PROGRAM="/bin/test -e
> /dev/evms/$env{DMNAME}", SYMLINK+="disk/by-label/$env{ID_FS_LABEL_SAFE}"
> 
> Although it seems to work, I'd like to hear your comments if you have any.
> 
> Also I've wrote a small article
> http://zaar.blogspot.com/2006/05/fine-grained-evms-device-nodes.html
> which explains why and how I've come to this.
> ---------------------------------------------------
> System: LFS-based, kernel 2.6.15, udev-0.8.1

Why does one ever need persistent names for DM devices? The default names (with
LVM2, this is something like /dev/mainvg/root) created by userspace packages are
already persistent enough to survive the move from old IDE drivers (that treat
IDE disks as /dev/hdX) to libata (which treats the same drivers as /dev/sdX)
without a single change to any configuration file.

Moreover, it is Just Wrong (TM) to have /dev/dm-X device nodes, because:

1) As already explained, userspace (e.g., dmsetup, vgmknodes, evms_activate,
etc.) already creates the correct nodes in the /dev/mapper directory and
symlinks to such nodes

2) The mere existence of /dev/dm-* breaks LILO (nodes in /dev/mapper are OK).

So, the only correct solution in this situation is implemented by Debian:

1) Prevent udev from creating device-mapper nodes at all, because it is the task
for other packages:

KERNEL="dm-[0-9]*",            NAME=""
KERNEL="device-mapper",        NAME="mapper/control"

2) Don't create persistent symlinks for DM devices, i.e. drop the line with
dmsetup from 60-persistent.rules (BTW, it uses SYMLINK="...", not SYMLINK+="..."
and is thus wrong again since it kills all other symlinks).

Kay: please remove the dmsetup rule from 60-persistent.rules.

-- 
Alexander E. Patrakov



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