LVM 'snapshot-merge' target not output via 'dmsetup targets' on RHEL 6

Solution Unverified - Updated -

Environment

  • Red Hat Enterprise Linux 6

Issue

  • The RHEL 6 man page for lvconvert specifies the following:
--merge
    Merges a snapshot into its origin volume or merges a raid1 image that has been split from its mirror with --trackchanges back into its mirror.

    To check if your kernel supports the snapshot merge feature, look for ’snapshot-merge’ in the output of ’dmsetup targets’.
  • However, the output of dmsetup-targets does not include the expected snapshot-merge target, as follows: 
[root@host ~]# dmsetup targets
mirror           v1.12.0
striped          v1.4.1
linear           v1.1.0
error            v1.0.1

Resolution

  • Either run an lvconvert --merge operation, which loads the module automatically, or load the module via modprobe, as follows:
[root@host ~]# modprobe dm-snapshot

Root Cause

  • The snapshot-merge target gets included when the dm-snapshot module is loaded, which can either be achieved through modprobe dm-snapshot or by running an lvconvert --merge operation. If the dm-snapshot module is present, then the target should load successfully.

  • The dm-snapshot module is included within the kernel for RHEL 6, as we can see in the following example:

$ modinfo ./lib/modules/2.6.32-279.11.1.el6.x86_64/kernel/drivers/md/dm-snapshot.ko
filename:       ./lib/modules/2.6.32-279.11.1.el6.x86_64/kernel/drivers/md/dm-snapshot.ko
license:        GPL
author:         Joe Thornber
description:    device-mapper snapshot target
srcversion:     8B0ADB0071DDCBBD2FD8D35
depends:        dm-mod
vermagic:       2.6.32-279.11.1.el6.x86_64 SMP mod_unload modversions
  • After loading the module (or running an lvconvert --merge operation) on RHEL 6, we see that the expected target is output when we run 'dmsetup targets':
# modprobe dm-snapshot

# dmsetup targets
snapshot-merge   v1.1.0
snapshot-origin  v1.7.1
snapshot         v1.10.0
mirror           v1.12.0
striped          v1.4.1
linear           v1.1.0
error            v1.0.1

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