Multipath devices using
Hello everyone,
I read the document "Red Hat Enterprise Linux 5 DM Multipath". It say that when have new devices under the control of DM-multipath, the new devices may be seen in places: /dev/mapper/mpathn, /dev/mpath/mpathn, /dev/dm-n. Do not use /dev/dm-n and use /dev/mapper to access the multipathd devices. Who can explain detail about it, please ? If I use /dev/dm-n to create file system and mount it to a mount point, then how do it affect ?
Thanks,
Responses
Guru
6222 points
Hello Dat Bach Ngoc,
Unlike /dev/mapper/mpathn (look at /var/lib/mpath/bindings) or /dev/mapper/your_names (/etc/multipath.conf), /dev/dm-n devices have no direct link to the world wide names of your scsi LUNs. /dev/dm-n devices are created at boot time in the order of detection. So when you added a scsi LUN with a wwn that change the ordering of the /dev/dm-n your filesystems will be mounted on a wrong mountpoint.
Very bad if the kernel filesystem gets mounted under /var and the root filesystem under /boot and var filesystem under /.
Your system will not boot anymore.
I advise use aliases like your_name in /etc/multipath.conf and use /dev/mapper/your_name as physical volumes for LVM and only use a partition /dev/mapper/your_namep1 for /boot
Kind regards,
Jan Gerrit Kootstra
KinAn,
please read Red Hat Enterprise Linux 7 DM Multipath / Configuration and Administration , quote:
When new devices are brought under the control of DM Multipath, the new devices may be seen in two different places under the /dev directory: /dev/mapper/mpathn and /dev/dm-n.
- The devices in /dev/mapper are created early in the boot process. Use these devices to access the multipathed devices, for example when creating logical volumes.
- Any devices of the form /dev/dm-n are for internal use only should never be used by the administrator directly.
Thanks.
PS: even usage of /dev/mapper directly, is the only valid case with Linux multipating, for example, with LVM, you should not use /dev/mapper/vg-volname, but /dev/vg/volname instead.
Welcome! Check out the Getting Started with Red Hat page for quick tours and guides for common tasks.