By the end of this article you should be able to answer the following questions:
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$ lvm help
LVM is all about creating/managing logical volumes. A logical volume (aka LV), can be thought of as a partition on a “virtual hdd”. The storage space of that logical volume can span across several block devices (i.e. actual partitions and hdds). LVs gives you a lot of flexibility to help you manage your various disk space more effectively. It also lets you create any size “virtual partition” that you want, e.g. you can have a 10TB virtual partition (in the form of a LV, which, behind the scenes is made up from several hdds/partitions)
LVM’s are a better alternative to using partitions for managing your storage space. Partitions are still necessary for some of the core directories, e.g. the
/boot directory needs to have it’s own primary partition or hdd. But after that it is better to have all other filesystems running on LVs.
Here are some of the advantages LVM offers over partitions:
- logical volumes are more flexible – LVs can be made up of several hdds/partitions, and you can extend/shrink LVs on the fly.
- easily replace hdds that are nearing the end of their lifetime
- Easily take backups in the form of LVM snapshots
- Easy to add new hdd’s to existing LVM setup
- Can create nearly unlimited LVs, i.e. not limited in the same way as with partitions, which you can only have up to 15 partitions per hdd.
Assuming that you already have a hdd or partition, then the overall process to creating LVs is as follows:
- Create new physical volumes (out of hdd’s and partitions)
- Add physical volumes to a volume group. If no VGs currently exist, then create one.
- Create a logical volume from a volume group.
You can treat an LV in the same way that you treat hdds/partitions, i.e. you can install a filesystem on it and mount it. However it has a couple of key differences:
- The mbr won’t store info about LVs
- You can’t mount
/booton a LV, since mbr won’t be able to pass over the boot process to an LV.
To create an LV volume, you have to take the following steps:
- Create “Physical Volumes” from block devices. (i.e. partitions and hdds) that have partition-id of 8e (for LVM). These block devices shouldn’t have been formatted (and therefore mounted). The act of creating a pv is like transforming a whole block device into a form of lego.
- Next you add the physical volume to a Volume Group. This is a bit like putting your legos into a lego bucket (i.e. VG), so that they are ready to be used for creating logical-volumes. The legos are often referred to as “physical extents”
- Next you create a “Logical Volume” by pulling out the required disk space from the volume-group.
- The LV can then be used like a partition, i.e. you can install a filestystem (format it) and mount it for use.
Note: you cannot install a filesystem on a physical volume or volume group…..they are just intermediary steps that leads to creating logical volumes. This means that physical volumes and volume groups, as standalones….are not that useful.
Here’s a summary of all the available LVM commands:
$ lvm help Available lvm commands: Use 'lvm help
' for more information devtypes Display recognised built-in block device types dumpconfig Dump configuration formats List available metadata formats help Display help for commands lvchange Change the attributes of logical volume(s) lvconvert Change logical volume layout lvcreate Create a logical volume lvdisplay Display information about a logical volume lvextend Add space to a logical volume lvmchange With the device mapper, this is obsolete and does nothing. lvmdiskscan List devices that may be used as physical volumes lvmsadc Collect activity data lvmsar Create activity report lvreduce Reduce the size of a logical volume lvremove Remove logical volume(s) from the system lvrename Rename a logical volume lvresize Resize a logical volume lvs Display information about logical volumes lvscan List all logical volumes in all volume groups pvchange Change attributes of physical volume(s) pvresize Resize physical volume(s) pvck Check the consistency of physical volume(s) pvcreate Initialize physical volume(s) for use by LVM pvdata Display the on-disk metadata for physical volume(s) pvdisplay Display various attributes of physical volume(s) pvmove Move extents from one physical volume to another pvremove Remove LVM label(s) from physical volume(s) pvs Display information about physical volumes pvscan List all physical volumes segtypes List available segment types tags List tags defined on this host vgcfgbackup Backup volume group configuration(s) vgcfgrestore Restore volume group configuration vgchange Change volume group attributes vgck Check the consistency of volume group(s) vgconvert Change volume group metadata format vgcreate Create a volume group vgdisplay Display volume group information vgexport Unregister volume group(s) from the system vgextend Add physical volumes to a volume group vgimport Register exported volume group with system vgmerge Merge volume groups vgmknodes Create the special files for volume group devices in /dev vgreduce Remove physical volume(s) from a volume group vgremove Remove volume group(s) vgrename Rename a volume group vgs Display information about volume groups vgscan Search for all volume groups vgsplit Move physical volumes into a new or existing volume group version Display software and driver version information