12.1. Multiple partitions on the same disk

LVM allows you to create PVs (physical volumes) out of almost any block device so, for example, the following are all valid commands and will work quite happily in an LVM environment:


# pvcreate /dev/sda1
# pvcreate /dev/sdf
# pvcreate /dev/hda8
# pvcreate /dev/hda6
# pvcreate /dev/md1
        

In a "normal" production system it is recommended that only one PV exists on a single real disk, for the following reasons:

  • Administrative convenience

    It's easier to keep track of the hardware in a system if each real disk only appears once. This becomes particularly true if a disk fails.

  • To avoid striping performance problems

    LVM can't tell that two PVs are on the same physical disk, so if you create a striped LV then the stripes could be on different partitions on the same disk resulting in a decrease in performance rather than an increase.

However it may be desirable to do this for some reasons:

  • Migration of existing system to LVM

    On a system with few disks it may be necessary to move data around partitions to do the conversion (see Section 13.8)

  • Splitting one big disk between Volume Groups

    If you have a very large disk and want to have more than one volume group for administrative purposes then it is necessary to partition the drive into more than one area.

If you do have a disk with more than one partition and both of those partitions are in the same volume group, take care to specify which partitions are to be included in a logical volume when creating striped volumes.

The recommended method of partitioning a disk is to create a single partition that covers the whole disk. This avoids any nasty accidents with whole disk drive device nodes and prevents the kernel warning about unknown partition types at boot-up.

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