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Revision as of 16:15, 24 October 2014 by Msimacek (talk | contribs) (Initial version of LVM plugin documentation)
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LVM plugin

Mock can use LVM as a backend for caching buildroots which is a bit faster than using root_cache and enables efficient snapshotting (copy-on-write). This feature is intended to be used by people who maintain a lot packages and find themselves waiting for mock to install the same set of BuildRequires over and over again. Mock uses LVM thin provisioning which means that one logical volume (called thinpool) can hold all thin logical volumes and snapshots used by all buildroots (you have to set it like that in the config) without each of them having fixed size. Thinpool is created by mock when it's starts initializing and after the buildroot is initialized, it creates a postinit snapshot which will be used as default. Default snapshot means that when you execute clean or start a new build without --no-clean option, mock will rollback to the state in default snapshot. As you install more packages you can create your own snapshots (usually for dependency chains that are common to many of your packages).

Expected workflow

You have multiple packages that all depend on something with big dependency chain, for example Java packages depend on maven-local, therefore it may be useful to create snapshot that would contain maven-local. With the next steps, you install the package and create the snapshot

 mock --install maven-local
 mock --snapshot mvn

Now there should be two snapshots - the initial one and the one you just created. You can verify it with the list-snapshots command (Note it also has short option -l)

 mock --list-snapshots
 Snapshots for fedora-20-x86_64:
 * mvn

The new one is marked with an asterisk, which means it will be used as the default snapshot to which --clean will rollback whenever you build another package. When you want to rebuild a package that doesn't use maven-local, you can use

 mock --rollback-to postinit

and the initial snapshot will be used for following builds. The mvn snapshot will still exist, so you can get back to it later using

 mock --rollback-to mvn

To get delete a snapshot completely, use mock --remove-snapshot mvn Mock will leave the buildroot volume mounted by default (you can override it in the config), so you can easily access the build directory. When you need to umount it, use

 mock --umount

To getting rid of LVM volumes completely, use

 mock --scrub lvm

This will delete all volumes belonging to the current config and also the thinpool if and only if there are no other volumes left by other configurations.


The plugin is distributed as separate subpackage mock-lvm because it pulls in additional dependencies which are not available on RHEL6. You need to specify a volume group which mock will use to create it's thinpool. Therefore you need to have some unoccupied space in your volume group, so you'll probably need to shrink some partition a bit. Mock won't touch anything else in the VG, so don't be afraid to use the VG you have for your system. It won't touch any other logical volumes in the VG that don't belong to it.


 config_opts['plugin_conf']['root_cache_enable'] = False
 config_opts['plugin_conf']['lvm_root_enable'] = True
 config_opts['plugin_conf']['lvm_root_opts'] = {
     'volume_group': 'my-volume-group',
     'size': '8G',
     'pool_name': 'mock',

Explanation: You need to disable root_cache - having two caches with the same contents would just slow you down. You need to specify a size for the thinpool. It can be shared across all mock buildroots so make sure it's big enough. Ideally there will be just one thinpool. Then specify name for the thinpool - all configs which have the same pool_name will share the thinpool, thus being more space-efficient. Make sure the name doesn't clash with existing volumes in your system (you can list existing volumes with lvs command).

Other configuration options

config_opts['plugin_conf']['lvm_root_opts']['poolmetadatasize'] - specifies separate size of the thinpool metadata. It needs to be big enough, thinpool with overflown metadata will become corrupted. When unspecified, the default value is determined by LVM

config_opts['plugin_conf']['lvm_root_opts']['umount_root'] - boolean option specifying whether the buildroot volume should stay mounted

after mock exits. Default is True config_opts['plugin_conf']['lvm_root_opts']['filesystem'] - filesystem name that will be used for the volume. It will use mkfs.$filesystem binary to create it. Default is ext4

config_opts['plugin_conf']['lvm_root_opts']['mkfs_command'] - the whole command for creating the filesystem that will get the volume path as an argument. When set, overrides above option.

config_opts['plugin_conf']['lvm_root_opts']['mkfs_args'] - additional arguments passed to mkfs command. Default is empty.

config_opts['plugin_conf']['lvm_root_opts']['mount_opts'] - will be passed to -o option of mount when mounting the volume. Default is empty.