Skip to content

what are unix logical volumes , physical volumes – knowledge article for the new oracle dba




you are a new oracle dba. whenever you are involved in a discussion with your colleagues or with your unix colleagues you hear discussions about unix physical volumes , logical volumes , physical extents(PE) etc. Below article explains in very easy terms about these terminologies so the next time you are involved in these discussions you can understand everything quite easily.

3. Basic principles

Ok, don’t let this scare you off, but LVM comes with a lot of jargon which you should understand lest you endanger your filesystems.

We start at the bottom, more or less.

The physical media

You should take the word ‘physical’ with a grain of salt, though we will initially assume it to be a simple hard disk, or a partition. Examples, /dev/hda, /dev/hda6, /dev/sda. You can turn any consecutive number of blocks on a block device into a …

Physical Volume (PV)

A PV is nothing more than a physical medium with some administrative data added to it – once you have added this, LVM will recognise it as a holder of …

Physical Extents (PE)

Physical Extents are like really big blocks, often with a size of megabytes. PEs can be assigned to a…

Volume Group

A VG is made up of a number of Physical Extents (which may have come from multiple Physical Volumes or hard drives). While it may be tempting to think of a VG as being made up of several hard drives (/dev/hda and /dev/sda for example), it’s more accurate to say that it contains PEs which are provided by these hard drives.

From this Volume Group, PEs can be assigned to a …

Logical Volume (LV)

Yes, we’re finally getting somewhere. A Logical Volume is the end result of our work, and it’s there that we store our information. This is equivalent to the historic idea of partitions.

As with a regular partition, on this Logical Volume you would typically build a …

Filesystem

This filesystem is whatever you want it to be: the standard ext2, ReiserFS, NWFS, XFS, JFX, NTFS, etc… To the linux kernel, there is no difference between a regular partition and a Logical Volume.

I’ve attempted some ASCII art which may help you visualise this.

A Physical Volume, containing Physical Extents:
 
  +-----[ Physical Volume ]------+
  | PE | PE | PE | PE | PE | PE  |
  +------------------------------+
 
A Volume Group, containing 2 Physical Volumes (PVs) with 6 Physical Extents:
 
  +------[ Volume Group ]-----------------+
  |  +--[PV]--------+  +--[PV]---------+  | 
  |  | PE | PE | PE |  | PE | PE | PE  |  |
  |  +--------------+  +---------------+  |
  +---------------------------------------+ 
 
We now further expand this:
 
  +------[ Volume Group ]-----------------+
  |  +--[PV]--------+  +--[PV]---------+  |
  |  | PE | PE | PE |  | PE | PE | PE  |  |
  |  +--+---+---+---+  +-+----+----+---+  |
  |     |   |   | +-----/     |    |      |
  |     |   |   | |           |    |      |
  |   +-+---+---+-+      +----+----+--+   |
  |   |  Logical  |      |  Logical   |   |
  |   |  Volume   |      |   Volume   |   |
  |   |           |      |            |   |
  |   |  /home    |      |    /var    |   |
  |   +-----------+      +------------+   |
  +---------------------------------------+

This shows us two filesystems, spanning two disks. The /home filesystem contains 4 Physical Extents, the /var filesystem 2.

bert hubert is writing a tool to represent LVM more visually, a screenshot is provided. Looks better than the ASCII art.

3.1 Show & Tell

Ok, this stuff is hard to assimilate (‘We are LVM of Borg…’), so here is a very annotated example of creating a Logical Volume. Do NOT paste this example onto your console because you WILL destroy data, unless it happens that on your computer /dev/hda3 and /dev/hdb2 aren’t used.

When in doubt, view the ASCIIgram above.

You should first set the partition types of /dev/hda3 and /dev/hdb2 to 0x8e, which is ‘Linux LVM’. Please note that your version of fdisk may not yet know this type, so it will be listed as ‘Unknown’:

# fdisk /dev/hda
 
Command (m for help): p
 
Disk /dev/hda: 255 heads, 63 sectors, 623 cylinders
Units = cylinders of 16065 * 512 bytes
 
   Device Boot    Start       End    Blocks   Id  System
/dev/hda1             1         2     16033+  83  Linux
/dev/hda2             3       600   4803435   83  Linux
/dev/hda3           601       607     56227+  83  Linux
/dev/hda4           608       614     56227+  83  Linux
 
Command (m for help): t
Partition number (1-4): 3
Hex code (type L to list codes): 8e
 
Command (m for help): p
 
Disk /dev/hda: 255 heads, 63 sectors, 623 cylinders
Units = cylinders of 16065 * 512 bytes
 
   Device Boot    Start       End    Blocks   Id  System
/dev/hda1             1         2     16033+  83  Linux
/dev/hda2             3       600   4803435   83  Linux
/dev/hda3           601       607     56227+  8e  Unknown
/dev/hda4           608       614     56227+  83  Linux
 
Command (m for help): w

We do the same for /dev/hdb2, but we don’t display it here. This is needed so that LVM is able to reconstruct things should you lose your configuration.

Now, this shouldn’t be necessary, but some computers require a reboot at this point. So if the following examples don’t work, try rebooting.

We then create our Physical Volumes, like this:

# pvcreate  /dev/hda3
pvcreate -- physical volume "/dev/hda3" successfully created
# pvcreate  /dev/hdb2
pvcreate -- physical volume "/dev/hdb2" successfully created

We than add these two PVs to a Volume Group called ‘test’:

# vgcreate test /dev/hdb2 /dev/hda3
vgcreate -- INFO: using default physical extent size 4 MB
vgcreate -- INFO: maximum logical volume size is 255.99 Gigabyte
vgcreate -- doing automatic backup of volume group "test"
vgcreate -- volume group "test" successfully created and activated

So we now have an empty Volume Group, let’s examine it a bit:

# vgdisplay -v test
--- Volume group ---
VG Name               test
VG Access             read/write
VG Status             available/resizable
VG #                  0
MAX LV                256
Cur LV                0
Open LV               0
MAX LV Size           255.99 GB
Max PV                256
Cur PV                2
Act PV                2
VG Size               184 MB
PE Size               4 MB
Total PE              46
Alloc PE / Size       0 / 0
Free  PE / Size       46 / 184 MB
 
--- No logical volumes defined in test ---
 
 
--- Physical volumes ---
PV Name (#)           /dev/hda3 (2)
PV Status             available / allocatable
Total PE / Free PE    13 / 13
 
PV Name (#)           /dev/hdb2 (1)
PV Status             available / allocatable
Total PE / Free PE    33 / 33

Lots of data here – most of it should be understandable by now. We see that there are no Logical Volumes defined, so we should work to remedy that. We try to generate a 50 megabyte volume called ‘HOWTO’ in the Volume Group ‘test’:

# lvcreate -L 50M -n HOWTO test 
lvcreate -- rounding up size to physical extent boundary "52 MB"
lvcreate -- doing automatic backup of "test"
lvcreate -- logical volume "/dev/test/HOWTO" successfully created

Ok, we’re nearly there, let’s make a filesystem:

# mke2fs /dev/test/HOWTO 
mke2fs 1.18, 11-Nov-1999 for EXT2 FS 0.5b, 95/08/09
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
13328 inodes, 53248 blocks
2662 blocks (5.00%) reserved for the super user
First data block=1
7 block groups
8192 blocks per group, 8192 fragments per group
1904 inodes per group
Superblock backups stored on blocks: 
        8193, 24577, 40961
 
Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done
# mount /dev/test/HOWTO /mnt
# ls /mnt
lost+found

And we’re done! Let’s review our Volume Group, because it should be filled up a bit by now:

# vgdisplay test -v
--- Volume group ---
VG Name               test
VG Access             read/write
VG Status             available/resizable
VG #                  0
MAX LV                256
Cur LV                1
Open LV               1
MAX LV Size           255.99 GB
Max PV                256
Cur PV                2
Act PV                2
VG Size               184 MB
PE Size               4 MB
Total PE              46
Alloc PE / Size       13 / 52 MB
Free  PE / Size       33 / 132 MB
 
--- Logical volume ---
LV Name               /dev/test/HOWTO
VG Name               test
LV Write Access       read/write
LV Status             available
LV #                  1
# open                1
LV Size               52 MB
Current LE            13
Allocated LE          13
Allocation            next free
Read ahead sectors    120
Block device          58:0
 
 
--- Physical volumes ---
PV Name (#)           /dev/hda3 (2)
PV Status             available / allocatable
Total PE / Free PE    13 / 13
 
PV Name (#)           /dev/hdb2 (1)
PV Status             available / allocatable
Total PE / Free PE    33 / 20

Well, it is. /dev/hda3 is completely unused, but /dev/hdb2 has 13 Physical Extents in use.

3.2 Active and Inactive: kernel space and user space

As with all decent operating systems, Linux is divided in two parts: kernel space and user space. Userspace is sometimes called userland, which would also be a good name for a theme park, ‘Userland’.

Discovery, creation and modification of things pertaining to Logical Volume Management is done in user space, and then communicated to the kernel. Once a volume group or logical volume is reported to the kernel, it is called ‘Active’. Certain changes can only be performed when an entity is active, others only when it is not.

 

VN:F [1.9.22_1171]
Rating: 0.0/10 (0 votes cast)
VN:F [1.9.22_1171]
Rating: 0 (from 0 votes)

Post a Comment

You must be logged in to post a comment.