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lvm



DESCRIPTION

       lvm is a logical volume manager for Linux.  It enables you
       to concatenate several physical volumes (hard disks  etc.)
       into  a  so  called  volume group (VG, see pvcreate(8) and
       vgcreate(8) ) forming a storage pool, like a virtual disk.
       IDE,  SCSI disks as well as multiple devices (MD) are sup­
       ported.  The storage capacity of a  volume  group  can  be
       divided into logical volumes (LVs), like virtual disk par­
       titions. The size of a logical volume is in  multiples  of
       physical extents (PEs, see lvcreate(8) ).
       The size of the physical extents can be configured at vol­
       ume group creation time. If a logical volume is too  small
       or  too  large  you  can  change its size at runtime ( see
       lvextend(8) and lvreduce(8) ).  lvcreate(8) can be used to
       create  snapshots  of  existing logical volumes (so called
       original logical volumes in this context) as well.
       Creating a snapshot logical volumes grants access  to  the
       contents  of  the original logical volume it is associated
       with and exposes the read only contents  at  the  creation
       time  of  the  snapshot. This is useful for backups or for
       keeping several versions of filesystems online.
       If you run out of space in a volume group it  is  possible
       to  add one or more pvcreate'd disks to the system and put
       them into an existing volume group (  see  vgextend(8)  ).
       The space on these new physical volumes can be dynamically
       added to logical volumes in that volume group ( see  lvex­
       tend(8) ).
       To  remove  a physical volume from the system you can move
       allocated logical extents to different physical volumes  (
       see  pvmove(8) ). After the pvmove the volume group can be
       reduced with the vgreduce(8) command.
       Inactive volume groups must be activated with  vgchange(8)
       before  use.   vgcreate(8) automatically activates a newly
       created volume group.


Abbreviations

       PV for physical volume, PE for  physical  extent,  VG  for
       volume  group,  LV  for logical volume, and LE for logical
       extent.


Command naming convention

       All command names corresponding to physical volumes  start
       with  pv,  all the ones concerned with volume groups start
       with vg and all for logical volumes with lv.  General pur­
       pose commands for the lvm as a whole start with lvm.


VGDA

       The volume group descriptor area (or VGDA for short) holds
       the necessary metadata to handle the LVM functionality. It
       is  stored  at  the beginning of each pvcreate'd disk.  It
       contains four parts: one PV descriptor, one VG descriptor,
       the LV descriptors and several PE descriptors. LE descrip­
       with  up  to  256  logical  volumes or anything in between
       these extreme examples.

       Depending on the physical extent size specified at  volume
       group creation time (see vgcreate(8) ), logical volumes of
       between a maximum of 512 Megabytes and 1 Petabyte  can  be
       created.   Actual  Linux  kernels  on IA32 limit these lvm
       possibilities to a maximum of 2 Terabytes per logical  and
       per  physical  volume as well. This enables you to have as
       much as 256 Terabytes under LVM control with all  possible
       128  scsi disk subsystems.  You can have up to 65534 logi­
       cal extents (on IA32) in a logical volume at the cost of 1
       Megabyte  in  kernel memory.  Physical volumes can have up
       to 65534 physical extents.


/proc filesystem support

       The operational state of active volume groups  with  their
       physical   and   logical  volumes  can  be  found  in  the
       /proc/lvm/ directory.  /proc/lvm/global contains a summary
       of  all  available  information regarding all VGs, LVs and
       PVs.  The two flags for PV status in brackets mean A/I for
       active/inactive  and  A/N  for allocatable or non-allocat­
       able.  The four flags for LV status in brackets  mean  A/I
       for  active/inactive, R/W for read-only or read/write, D/C
       for discontiguous or contiguous  and  L/S  for  linear  or
       striped.   S  can  optionally be followed by the number of
       stripes in the set.  At /proc/lvm/VGs/ starts a  subdirec­
       tory  hierarchy containing information about every VG in a
       different subdirectory named /proc/lvm/VGs/VolumeGroupName
       where  VolumeGroupName  stands  for  an arbitrary VG name.
       /proc/lvm/VGs/VolumeGroupName/ in turn holds a file  group
       containing  summary  information  for  the  VG as a total.
       /proc/lvm/VGs/VolumeGroupName/LVs/LogicalVolumeName  holds
       information  for  an  arbitrary LV named LogicalVolumeName
       /proc/lvm/VGs/VolumeGroupName/PVs/PhysicalVolumeName  con­
       tains  information for an arbitrary PV named PhysicalVolu­
       meName.   All  of  the  information  in  the  files  below
       /proc/lvm/VGs/ is presented in attribute/value pairs to be
       easyly parsable.


Examples

       We have disk partitions /dev/sda3, /dev/sdb1 and /dev/hda2
       free  for  use  and  want  to  create a volume group named
       "test_vg".  Steps required:

       1. Change partition type for these 3  partitions  to  0x8e
       with  fdisk.  (see pvcreate(8): 0x8e identifies LVM parti­
       tions)

       2. pvcreate /dev/sda3 /dev/sdb1 /dev/hda2

       Now let's rock and roll.  For example create a file system
       with  "mkfs  -t ext2 /dev/test_vg/my_test_lv" and mount it
       with "mount /dev/test_vg/my_test_lv /usr1"


See also

       e2fsadm(8), lvchange(8), lvcreate(8), lvdisplay(8),
       lvextend(8), lvmchange(8), lvmdiskscan(8),
       lvmcreate_initrd(8), lvmsadc(8), lvmsar(8),
       lvreduce(8), lvremove(8), lvrename(8),
       lvscan(8), pvchange(8), pvcreate(8), pvdata(8),
       pvdisplay(8), pvmove(8), pvscan(8), vgcfgbackup(8),
       vgcfgrestore(8), vgchange(8), vgck(8), vgcreate(8),
       vgdisplay(8), vgexport(8), vgextend(8), vgimport(8),
       vgmerge(8), vgmknodes(8), vgreduce(8), vgremove(8),
       vgrename(8), vgscan(8), vgsplit(8)


AUTHOR

       Heinz Mauelshagen <Linux-LVM@Sistina.com>

Heinz Mauelshagen           LVM TOOLS                      LVM(8)
  




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