Logical Volume Manager Abstract                                      12/01/2001

Goals:
------
Implement a very flexible virtual disk subsystem to handle disk storage.
Online allocation and relocation of storage.
Online extension and reduction of storage.


Function:
---------
The Logical Volume Manager (LVM) adds an additional layer between
the physical peripherals and the i/o interface in the kernel to
get a logical view of disks.

This allows the concatenation of several disks (so-called physical volumes
or PVs) to form a storage pool (so-called Volume Group or VG) with
allocation units called physical extents (called PE).

Parts out of this VG then can be allocated in form of
so-called Logical Volumes or LVs in units called logical extents or LEs.
Each logical extent is mapped to a corresponding physical extent 
of equal size. These physical extents are mapped to offsets and blocks
on the disk(s).

The LVs can then be used through device special files similar to
/dev/sd[a-z]* or /dev/hd[a-z]* named /dev/VolumeGroupName/LogicalVolumeName.

But going beyond this, you are able to extend or reduce VGs AND LVs at runtime.


Concept:
--------
The configuration information for the physical volume, volume group and
logical volume(s) is stored on each physical volume and in automatically
created backup files, which are stored in the /etc/lvmtab.d directory.

The config area on the disk(s) is called Volume Group Descriptor Area or VGDA.

A LVM driver holds mapping tables between the LEs of LVs and the PEs of PVs.
These tables are created/updated/deleted by superuser LVM commands.

The main mapping function of the driver is called with a logical block
in a LV from functions in /usr/src/linux/drivers/block/ll_rw_blk.c
(in functions ll_rw_block() and ll_rw_swap_file() ) and looks up the
corresponding physical block/disk pair in a table.
Then it returns this pair to the calling ll_rw_*() function causing a
physical i/o request to the disk block(s) to be queued.



Example:
--------
If the capacity of a LV gets too small and your VG containing this LV is full,
you could add another PV to that VG and simply extend the LV afterwards.
If you reduce or delete a LV you can use the freed capacity for different
LVs of the same VG.


The above scenario looks like this:

     /------------------------------------------\
     |  /--------\      VG 1      /--------\    |
     |  |        |                |        |    |
     |  |  PV 1  |     ......     | PV n   |    |
     |  |    /-----------------------\     |    |
     |  |    \-------LV 1------------/     |    |
     |  \--------/                \--------/    |
     \------------------------------------------/

PV 1 could be /dev/sdc1
PV n could be /dev/sde1
VG 1 could be vg00
LV 1 could be /dev/test_vg/test_lv




The configuration steps for getting the above scenario are:

1. after installing LVM do an "insmod lvm" or
   setup kerneld/kmod to load it automatic (see INSTALL)

2. set up partitions (#1) on both disks with partition type 0x8e
   (I used this type to avoid using Linux primary partitions etc.)

3. do a "pvcreate /dev/sd[ce]1"
   For testing purposes you can use more than one
   primary and/or extended partition on a disk.
   Don't do that for normal LVM operation for performance reasons.
   Unless you have to, don't stripe logical volumes over physical volumes
   associated to partitions on the same disk.

4. do a "vgcreate test_vg /dev/sd[ce]1"
   (vgcreate activates the volume group too)

5. do a "lvcreate -L100 -ntest_lv test_vg" to get a 100MB linear LV
   or a "lvcreate -i2 -I4 -l100 -nanother_test_lv test_vg" to get a
   100 LE large logical volume with 2 stripes and stripe size 4 KB.

6. use created LVs as you want to.
   For example generate a filesystem in one with "mke2fs /dev/test_vg/test_lv"
   and mount it.



Overview and concept of commands:
---------------------------------
I grouped and named LVM commands analogous to HP's.
So the commands for physical volume handling all start with pv,
those for volume group handling start with vg and the ones for
logical volumes start with lv.


e2fsadm      - administration wrapper for logical volume including filesystem
               resizing for lvextend, lvreduce, e2fsck and resize2fs

lvchange     - change attributes of a logical volume
lvcreate     - create a logical volume
lvdisplay    - display logical volume config data
lvextend     - extend a logical volume in size
lvreduce     - reduce a logical volume in size
lvremove     - remove a logical volume
lvrename     - rename an inactive logical volume
lvscan       - find all existing logical volumes

lvmchange    - emergency program to change attributes of the LVM
lvmdiskscan  - scan all disks / partitions and multiple devices and list them
lvmsadc      - statistic data collector
lvmsar       - statistic data reporter

pvchange     - change attributes of physical volumes
pvcreate     - create a physical volume
pvdata       - debug list physical volume group descriptor area
pvdisplay    - display physical volume config information
pvmove       - move logical extents to a different physical volume
pvscan       - find all existing physical volumes

vgcfgbackup  - backup all volume group descriptor areas
vgcfgrestore - restore volume group descriptor area(s) to disk(s)
vgchange     - activate/deactivate volume group(s)
vgck         - check volume group descriptor area for consistency
vgcreate     - create a volume group from physical volume(s)
vgdisplay    - display volume group config information
vgexport     - export volume group (make it unknown to the system)
vgextend     - extend a volume group by one or more physical volumes
vgimport     - import a volume group (make it known to the/another system)
vgmerge      - merge two volume groups into one
vgmknodes    - create volume group directory with all logical volume specials
vgreduce     - reduce a volume group by one or more empty physical volume(s)
vgremove     - remove an empty volume group
vgrename     - rename an inactive volume group
vgscan       - scan for volume groups
vgsplit      - split one volume group into two



Example LVM session output:
---------------------------

# create physical volumes on 9 SCSI disk primary partition 1
pvcreate /dev/sd[b-eg-k]1
pvcreate -- /dev/sdb1 has an invalid physical volume identifier
pvcreate -- physical volume on /dev/sdb1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdc1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdd1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sde1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdg1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdh1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdi1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdj1 successfully created
pvcreate -- reinitializing physical volume
pvcreate -- physical volume on /dev/sdk1 successfully created



# create a volume group with default physical extent size
# from these physical volumes
vgcreate my_first_vg /dev/sd[b-eg-k]1
vgcreate -- INFO: using default physical extent size 4096 KB
vgcreate -- INFO: maximum logical volume size is 256 GB
vgcreate -- doing automatic backup of vg05
vgcreate -- volume group my_first_vg successfully created


# Oops ;-)
# Don't like the limitations caused by default physical extent size
# --> deactivate and delete volume
vgchange -an my_first_vg
vgchange -- my_first_vg successfully deactivated

vgremove my_first_vg
vgremove -- volume group my_first_vg successfully removed



# create a volume group with physical extent size of 8192 KB
# from these physical volumes
vgcreate -s 8192 my_first_vg /dev/sd[b-eg-k]1
vgcreate -- INFO: maximum logical volume size is 512 GB
vgcreate -- doing automatic backup of my_first_vg
vgcreate -- volume group my_first_vg successfully created



# display volume group config
vgdisplay my_first_vg
--- Volume group ---
VG Name               my_first_vg
VG Write Access       read/write
VG Status             available
VG #                  1
MAX LV                31
Cur LV                0
Open LV               0
MAX LV Size           512 GB
Max PV                256
Cur PV                9
Act PV                9
VG Size               12636 MB
PE Size               8192 KB
Total PE              1579
Alloc PE / Size       0 / 0 KB
Free  PE / Size       1579 / 12632 MB


# do it again Sam but verbose
vgdisplay -v my_first_vg
--- Volume group ---
VG Name               my_first_vg
VG Write Access       read/write
VG Status             available
VG #                  1
MAX LV                31
Cur LV                0
Open LV               0
MAX LV Size           512 GB
Max PV                256
Cur PV                9
Act PV                9
VG Size               12636 MB
PE Size               8192 KB
Total PE              1579
Alloc PE / Size       0 / 0 KB
Free  PE / Size       1579 / 12636 MB

--- No logical volumes defined in my_first_vg ---


--- Physical volumes ---
PV Name (#)           /dev/sdb1 (1)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sdc1 (2)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sdd1 (3)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sde1 (4)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sdg1 (5)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sdh1 (6)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sdi1 (7)
PV Status             available / allocatable
Total PE / Free PE    131 / 131

PV Name (#)           /dev/sdj1 (8)
PV Status             available / allocatable
Total PE / Free PE    125 / 125

PV Name (#)           /dev/sdk1 (9)
PV Status             available / allocatable
Total PE / Free PE    537 / 537



# create a linear physical volume with all space of the volume group
lvcreate -l1579 my_first_vg
lvcreate -- doing automatic backup of my_first_vg
lvcreate -- logical volume /dev/my_first_vg/lvol1 successfully created



# create an ext2fs on newly created logical volume
mke2fs /dev/my_first_vg/lvol1
mke2fs 1.10, 24-Apr-97 for EXT2 FS 0.5b, 95/08/09
Linux ext2 filesystem format
Filesystem label=
3235840 inodes, 12939264 blocks
646963 blocks (5.00%) reserved for the super user
First data block=1
Block size=1024 (log=0)
Fragment size=1024 (log=0)
1580 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks: 
	8193, 16385, 24577, 32769, 40961, 49153, 57345, 65537, 73729, 
	81921, 90113, 98305, 106497, 114689, 122881, 131073, 139265, 147457, 
<SNIP>
Writing superblocks and filesystem accounting information: done
0.66user 26.55system 3:06.62elapsed 14%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (97major+522minor)pagefaults 0swaps



# and mount it (not very exiting :-) )
mount /dev/my_first_vg/lvol1 /mnt

