Device-mapper

Device-mapper is a new infrastructure in the Linux 2.6 kernel that provides a generic way to create virtual layers of block devices that can do different things on top of real block devices like striping, concatenation, mirroring, snapshotting, etc...

The Device-mapper was introduced only in version 2.6 of the linux kernel. It is required by LVM2. The original LVM (included in 2.4 kernels) does not use it.

If you intend to use Device-mapper, as well as including it in your kernel, you should install the userspace configuration tool (dmsetup) and library (libdevmapper).

Multipath tools are available from ftp://sources.redhat.com/pub/dm/multipath-tools with documentation at http://christophe.varoqui.free.fr/.

Chapter 1. Device Mapper Multipathing

Multipath Devices in Logical Volumes

After creating multipath devices, you can use the multipath device names just as you would use a physical device name when creating an LVM physical volume. For example, if /dev/mapper/mpath0 is the name of a multipath device, the following command will mark /dev/mapper/mpath0 as a physical volume.

pvcreate /dev/mapper/mpath0

You can use the resulting LVM physical device when you create an LVM volume group just as you would use any other LVM physical device.

When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you should include filters in the lvm.conf to exclude the disks that underlie the multipath devices. This is because if the array automatically changes the active path to the passive path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if these devices are not filtered. For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs.

To filter all SCSI devices in the multipath configuration file (lvm.conf), include the following filter in the devices section of the file.

filter = [ "r/disk/", "r/sd.*/", "a/.*/" ]

Releases

The userspace code (dmsetup and libdevmapper) is now maintained alongside the LVM2 source available from http://sources.redhat.com/lvm2/. To build / install it without LVM2 use 'make device-mapper' / 'make device-mapper_install'.

Kernel patches under discussion are captured by patchwork from the dm-devel mailing list and stored at http://patchwork.kernel.org/project/dm-devel/list/?state=*.
A quilt tree of patches being prepared for inclusion upstream is maintained at http://www.kernel.org/pub/linux/kernel/people/agk/patches/2.6/editing/
with archived versions at http://www.kernel.org/pub/linux/kernel/people/agk/patches/2.6/

Old kernel source code patches for 2.4 and 2.6 kernels are still available from http://sources.redhat.com/dm/patches.html.

Old source code tarballs of the userspace tool and library are still available from ftp://sources.redhat.com/pub/dm/.
These also include kernel patches for some specific 2.4 kernels.

Mailing lists

dm-devel is the mailing list for any device-mapper-related questions and discussion.
Subscribe to this from https://www.redhat.com/mailman/listinfo/dm-devel.
The list archives are at https://www.redhat.com/archives/dm-devel/.
The mailing list address is [email protected].

Whenever the development source code repository is updated, email is sent to the dm-cvs mailing list. This list is run using ezmlm.
To subscribe, send an email to [email protected] or [email protected].
You can read the list archives at http://sources.redhat.com/ml/dm-cvs/

The encryption target has its own mailing list archived at http://news.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt.
To subscribe, send a blank email to [email protected].

Source code

To access the old source code repository using CVS:
cvs -d :pserver:[email protected]:/cvs/dm login cvs
cvs -d :pserver:[email protected]:/cvs/dm checkout device-mapper
The password is cvs.

There is a web interface to CVS at http://sources.redhat.com/cgi-bin/cvsweb.cgi/device-mapper/?cvsroot=dm.

Multipath Bug Reporting

As well as using the dm-devel mailing list, we are using bugzilla.redhat.com to record multipath bugs and feature requests.
Display outstanding multipath bugzilla entries. Report multipath bug. Enter multipath feature request.

NEWS CONTENTS

• 200208 : dev-dm-0 ( dev-dm-0, )

Old News

dev-dm-0

I am having problems with Mondo Rescue. I have two drives. One 20 gig, one 300 gig.

Initializing...
See /var/log/mondo-archive.log for details of backup run.
Checking sanity of your Linux distribution
Disk /dev/dm-0 doesn't contain a valid partition table
Disk /dev/dm-1 doesn't contain a valid partition table
Disk /dev/dm-2 doesn't contain a valid partition table
Disk /dev/dm-3 doesn't contain a valid partition table
Done.
Fatal error... output folder does not exist - please create it
---FATALERROR--- output folder does not exist - please create it
If you require technical support, please contact the mailing list.
See http://www.mondorescue.org for details.
The list's members can help you, if you attach that file to your e-mail.
Log file: /var/log/mondo-archive.log
FYI, I have gzipped the log and saved it to /tmp/MA.log.gz
Mondo has aborted.
Execution run ended; result=254
Type 'less /var/log/mondo-archive.log' to see the output log

fdisk -l

Disk /dev/dm-0: 6308 MB, 6308233216 bytes
255 heads, 63 sectors/track, 766 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/dm-0 doesn't contain a valid partition table

Disk /dev/dm-1: 973 MB, 973078528 bytes
255 heads, 63 sectors/track, 118 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/dm-1 doesn't contain a valid partition table

Disk /dev/dm-2: 299.5 GB, 299573968896 bytes
255 heads, 63 sectors/track, 36421 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/dm-2 doesn't contain a valid partition table

Disk /dev/dm-3: 22.4 GB, 22481469440 bytes
255 heads, 63 sectors/track, 2733 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/dm-3 doesn't contain a valid partition table

error, disk, horrifying, brokensystem, broken

12-29-2006, 05:24 AM		#2
dombrowsky Member Registered: Dec 2005 Location: New York Distribution: Debian/GNU Posts: 229 Thanked: 0	looks like the data on your disks is broken. period. not sure what the question is here.

02-07-2008, 09:00 PM		#3
systemnyc LQ Newbie Registered: Sep 2006 Posts: 4 Thanked: 0	Same problem as Bic Disk /dev/sda: 160.0 GB, 160041885696 bytes 116 heads, 43 sectors/track, 62666 cylinders, total 312581808 sectors Units = sectors of 1 * 512 = 512 bytes Disk identifier: 0xfd7382ed Device Boot Start End Blocks Id System /dev/sda1 * 43 188067551 94033754+ 7 HPFS/NTFS /dev/sda2 188067552 188476567 204508 83 Linux /dev/sda3 188476568 312578007 62050720 8e Linux LVM Disk /dev/dm-0: 62.4 GB, 62444797952 bytes 255 heads, 63 sectors/track, 7591 cylinders, total 121962496 sectors Units = sectors of 1 * 512 = 512 bytes Disk identifier: 0x00000000 Disk /dev/dm-0 doesn't contain a valid partition table Disk /dev/dm-1: 1040 MB, 1040187392 bytes 255 heads, 63 sectors/track, 126 cylinders, total 2031616 sectors Units = sectors of 1 * 512 = 512 bytes Disk identifier: 0x30307800 Disk /dev/dm-1 doesn't contain a valid partition table Okay, now the question I have is; 1. what does this mean, and 2. how do I repair the partition?

02-08-2008, 07:20 AM		#4
bic Member Registered: Jan 2005 Distribution: RH9, MythDora 3.2, RHEL 4 Posts: 77 Thanked: 1 Original Poster	I don't remember what I did to fix it. I use g4u now instead. http://www.feyrer.de/g4u/

02-25-2008, 11:52 AM		#5
darthwonka LQ Newbie Registered: Sep 2006 Distribution: Debian Testing, Fedora 8, CentOS5, Ubuntu Posts: 1 Thanked: 0	Is this really a problem? Maybe fdisk doesn't know how to talk LVM. The errors I get with fdisk all point to the logical volumes.

02-27-2008, 03:32 AM		#6
v00d00101 Member Registered: Jun 2003 Location: UK Distribution: Fedora 8, Centos 5.1 Posts: 480 Thanked: 0	Isnt /dev/dm-x a device mapper virtual mount point? I only use those for mounting flash drives, and mapping encrypted partitions. Sorry, i dont do LVM anymore, after a small problem lost me 300GB of data. Its much easier to backup. Last edited by v00d00101; 02-27-2008 at 03:34 AM..

04-18-2008, 07:20 AM		#7
kotao_78 LQ Newbie Registered: Sep 2006 Posts: 16 Thanked: 0	How to remove them? How to remove them?

04-23-2008, 07:41 PM		#8
stoat Member Registered: May 2007 Posts: 68 Thanked: 0	You don't remove them. Just don't look at them. Seriously. Those are device mapper devices. They usually are associated with LVM and correspond to the logical volumes. But those devices can also appear in the fdisk report when things such as flash drives and camera memory are connected to the computer. Anyway, they're normal. Last edited by stoat; 10-22-2008 at 04:11 PM..

05-09-2008, 07:31 AM		#9
jschlesi LQ Newbie Registered: Oct 2006 Location: Brookline, MA, USA Distribution: Fedora C5 Posts: 11 Thanked: 0	This has been very helpful to me. I found this thread by Goggle on dm-0 because I also got the no partition table error message. Here is what I think: When the programs fdisk and sfdisk are run with the option -l and no argument, e.g. # /sbin/fdisk -l they look for all devices that can have cylinders, heads, sectors, etc. If they find such a device, they output that information to standard output and they output the partition table to standard output. If there is no partition table, they have an error message (also standard output). One can see this by piping to 'less', e.g. # /sbin/fdisk -l | less the error messages disappear. /dev/dm-0 ... /dev/dm3 on my fedora C5 system seem to be device mappers associated with LVM. RAID might also require device mappers.

10-22-2008, 12:30 PM		#10
cheber LQ Newbie Registered: Oct 2008 Posts: 5 Thanked: 0	Well, I got the "/dev/dm-0" messages when I had simply removed partitions containing LVM. I then removed the VGs and PVs and I didn't get the messeges anymore when running "fdisk -l". Last edited by cheber; 10-22-2008 at 12:31 PM..

Recommended Links

Device mapper - Wikipedia, the free encyclopedia

device-mapper-udev-crs-asm%20rh4

There are README, INSTALL, WHATS_NEW_DM libdevmapper.h and dmsetup man page in the LVM2 source package.

The LVM HOWTO is at http://www.tldp.org/HOWTO/LVM-HOWTO/index.html.

LVM2: http://sources.redhat.com/lvm2/
EVMS: http://evms.sourceforge.net/
The original LVM: http://sources.redhat.com/lvm/

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