2009-10-18.

I've never yet really seen an advantage (for me) in LVM, /except/,
as recently written about, the use of its device-mapper parent 
for implementing an encrypted disk partition.

Still, a little playing about could be interesting, now that I've
reserved a large partition in the desktop's disk for just this 
purpose (and ultimately for a bit of temporary storage space).

[And, addendum after a few minutes: even for this rather 
artificial use, of dividing a largish partition into parts,
doing this with good flexibility of resizing, subject of 
course to the filesystems' ease of resizing, does seem 
convenient compared to having some hundreds of GB of 
disparate groups of data all needing backup before changing
a filesystem.]


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The lvm command is nicely designed: it does several jobs, each 
a worthily-featured command in itself, and can be called either
with an argument to say which command to act as, or called by 
this named (e.g. run a symlink), or if lvm is run alone it 
gives an interactive shell in which these commands can be used.


We've got /dev/sda4, a `Linux LVM' dos-partition, available
as the physical volume.

# fdisk -l /dev/sda

Disk /dev/sda: 320.0 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1        3648    29302528+  83  Linux
/dev/sda2            3649        4147     4008217+  82  Linux swap / Solaris
/dev/sda3            4148        7066    23446867+  a5  FreeBSD
/dev/sda4            7067       38913   255811027+  8e  Linux LVM


There's a fair old bit of DM (device-mapper) stuff in the kernel:

# cat /proc/config.gz  | gunzip | grep _DM_
CONFIG_DM_DEBUG=y
CONFIG_DM_CRYPT=m
CONFIG_DM_SNAPSHOT=m
CONFIG_DM_MIRROR=m
# CONFIG_DM_LOG_USERSPACE is not set
CONFIG_DM_ZERO=m
CONFIG_DM_MULTIPATH=m
# CONFIG_DM_MULTIPATH_QL is not set
# CONFIG_DM_MULTIPATH_ST is not set
CONFIG_DM_DELAY=m
CONFIG_DM_UEVENT=y
CONFIG_BLK_DEV_DM_BBR=m


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Make some logical volumes.


# lvm pvcreate /dev/sda4
  Physical volume "/dev/sda4" successfully created

# lvm vgcreate space  /dev/sda4
  Volume group "space" successfully created

# lvm lvcreate -L 120G -n user space
  Logical volume "user" created

# mkfs.ext3 /dev/mapper/space-user


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Find out more:

# lvm pvdisplay
  --- Physical volume ---
  PV Name               /dev/sda4
  VG Name               space
  PV Size               243.96 GB / not usable 3.46 MB
  Allocatable           yes
  PE Size (KByte)       4096
  Total PE              62453
  Free PE               62453
  Allocated PE          0
  PV UUID               YJ7i01-1kvb-uC2X-AyiN-GRA2-bG3e-YYo0Eo

# lvm vgdisplay
  --- Volume group ---
  VG Name               space
  System ID
  Format                lvm2
  Metadata Areas        2
  Metadata Sequence No  1
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                0
  Open LV               0
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               243.96 GB
  PE Size               4.00 MB
  Total PE              62453
  Alloc PE / Size       0 / 0
  Free  PE / Size       62453 / 243.96 GB
  VG UUID               tWc3Np-HbU1-AHrf-Luz3-eITA-KzAv-Z64Kdw

# lvm lvdisplay space/user
  --- Logical volume ---
  LV Name                /dev/space/user
  VG Name                space
  LV UUID                NSaLfR-J4FK-mtgq-PDFM-WRGy-PrHD-llcr46
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                120.00 GB
  Current LE             30720
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1

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