KNOW HOW
LVM: Enterprise computing with
the Linux Logical Volume Manager
VARIABLE
DIVISION
A Logical Volume
ne of the most important requirements in the thereby considerably cuts down on the costs of
field of professional IT is to be able to learning, my own implementation is largely based on
Manager (LVM)
Oreconfigure computer systems online and this. Incidentally, LVM was originally developed by
makes it possible to
without halting operations. In this regard, logical IBM and adopted by the OSF (now the Open Group).
volume management plays a major role. The The LVM implementation in HP-UX is based on this.
adapt disk capacity
advantages are obvious: time and costs are saved, as To be able to use the logic block devices provided
to dynamically
back up and restore tasks are dispensed with, and by the LVs under Linux too, and to set up filesystems
applications don t have to be interrupted, so there on them, an expansion of the Linux kernel is
changing
are no expensive system stoppages. necessary. This is done by means of a device driver.
requirements while
This is achieved by decoupling block devices and
physical disk partitions. The latter, as physical storage Queuing magic
the system is still in
media (Physical Volumes, PVs for short) form the In this article, I will limit myself to the principle and
use. Heinz
lowest level of a three-level architecture. One or more the application of LVM under Linux 2.4. If anyone
PVs are combined on the second level into virtual would like to delve somewhat deeper, I would advise
Mauelshagen
disks (Volume Groups, VGs). The full memory capacity studying the source code of the kernel and Linux LVM
explains why LVM is
(minus a small metadata portion per PV) can be assuming you have knowledge of C. Hence this
assigned to virtual partitions in the third level (Logical article will also make references to these sources.
indispensable for
Volumes, LVs). The LVs are addressed as regular Linux Under Linux 2.4, in addition to elementary
business-critical
block device files, so that any filesystems can be set functions such as open, close, read and write, a block
up on them (see Figure 1). device driver registers a make request function, which
applications
is invoked by a central function of the Linux block
HP-UX as godfather device layer (see /usr/ src/ linux/ drivers/ block/
When LVM is used, physical ll_rw_blk.c; functions ll_rw_block, submit_bh and
disks can be added to a system generic_make_request), before an I/O request is
and the capacity of existing ranked in a device-specific queue.
volumes can be assigned to Queues serve the best possible processing of I/O
them. After many years of requests, by holding these for a (very) short time in
experience with commercial the queue (device plugging). This is in order to put
LVM variants of HP, IBM, Sun them in the best possible sequence before they are
and Veritas, my fingers were passed on to the device for processing (such as an
itching to expand Linux by LVM ATA-adapter) (device-unplugging).
functionalities. The LVM project Since LVM has to convert between logical and
started in February 1997, and subordinate (physical) devices, it implements a re-
version 0.1 was launched in July mapping driver. This contains its own make request
1997. After some wide-ranging function and registers it, so that before submitting a
functional expansions in the request (call up of generic_make_request in /usr/ src/
past few years, version 1.0 was linux/ drivers/ block/ ll_rw_blk.c), it can perform
released in August of this year. manipulations on its administration data.
Since the LVM in HP-UX The administration data relevant to us is in the
displays a highly intuitive buffer_ head structure, which is set up by the kernel
Figure 1: The three-level memory
architecture of the Linux LVM command line interface and for each buffer containing I/O data. All I/O data
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LINUX MAGAZINE Issue 16 " 2002
KNOW HOW
LVM application
Once all the LVM elements (PV, VG, LV, LE and PE)
have been pre-set, it s the turn of the application
itself. Similarly to PVs, whose names are defined by
the device files issued by Linux (such as /dev/ hdb2),
VGs and LVs also receive a name when they are re-
made. VG names appear in the form of subdirectories
in /dev/ and LV names appear as block device files in
the VG subdirectories.
The user interface of the Linux LVM is implemented
as a CLI (Command Line Interface) with 35
commands, which correspond to the three levels of
the memory architecture. All commands for
manipulation of the PVs begin with pv; all those for
the VGs with vg; and those belonging to LVs with lv.
Since almost every level is involved with creating,
removing, displaying, extending, reducing,
renaming, scanning or changing attributes, most of
the command names are produced from a
combination of the prefixes pv, vg or lv with these
abilities (Table 1).
In addition to these, there are commands for
backing up and restoring the metadata stored on
Figure 2: Example of the mapping of
an LV onto two different PVs the PVs; to move VGs from one system to another;
to combine two VGs into one or to split up one
buffers in the disk cache which the kernel VG; to move LEs or LVs of assigned PEs; and to
maintains and dynamically adapts in size for change the size of an LV including the Ext2
performance reasons have a buffer_head structure. filesystem (Table 2).
Linux 2.4, unlike Version 2.2 and predecessors, Don t be scared off by the number of commands
now only performs caching in a page cache and uses at this point, since only three commands are
buffer_ head only at the interface with the block necessary to create the first LV: pvcreate, vgcreate
device layer, whose central function is ll_rw_ block. and lvcreate. There are manuals available for all the
buffer_head (see /usr/ src/ linux/ include/ linux/ fs.h)
has, in addition to several members, a real sector
address b_rsector and the address of a real device
Table 1: Basic LVM commands
b_rdev as content.
pvcreate Create a PV
After opening a logical volume (LV) by mke2fs
pvdisplay Display the attributes of PVs
there follows some read and write accesses, so as to
pvscan Scan for existing PVs
save the Ext2 filesystem structures on it. The
pvchange Alter attributes of PVs
ll_rw_block calls executed at this point lead directly to
vgcreate Create a new VG
the invocation of the LVM driver Make-Request
vgremove Remove an empty VG without LVs
function, which is called the lvm_make_request_fn
vgextend Extend a VG by additional PVs
and is defined in /usr/ src/ linux/ drivers/ md/ lvm.c.
vgreduce Reduce a VG by empty PVs
The function invoked by lvm_make_request_fn,
vgdisplay Display the attributes of VGs
lvm_map requires a table in which the addresses of
vgrename Rename a VG
the devices namely those of the physical volumes
vgscan Scan for existing VGs
and the sector addresses thereon are listed.
vgchange Change attributes and activate/deactivate VGs
To avoid the need for a table entry for every
lvcreate Create an LV
individual sector which would end up as a gigantic
lvremove Remove inactive LVs
table a number of sectors lying one behind the
lvextend Extend an LV
other are combined into physical extents (PE) and
lvreduce Reduce an LV
assigned one to one in the logical address space of
lvdisplay Display the attributes of LVs
LV, as logical extents (LE) of the same size. The
lvrename Rename an LV
mapping table thus contains an entry for each
lvscan Scan for all existing LVs
assigned PE, describing the address (b_rdev) and the
lvchange Change attributes of an LV
real start sector (b_rsector) on the respective PV
(Figure 2).
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Issue 16 " 2002 LINUX MAGAZINE
KNOW HOW
lvcreate -n melv -L100 mevg
Table 2: Extended LVM commands
which creates an LV named melv (my first LV) with
pvdata Debug displays of the attributes of PVs
100Mb. This LV has the device name /dev/ mevg/
pvmove Move LV data online
melv. Via
vgcfgbackup Perform back up of metadata of VGs
vgcfgrestore Restore metadata on PVs of a VG
mke2fs /dev/mevg/melv
vgck Check consistency of metadata of VGs
vgexport Log off a VG, in order to move its PVs to another system
a filesystem can now be installed, which can be
vgimport Make moved VG known to the destination system
mounted as usual in any directory of your choice that
vgmerge Combine two VGs into one
uses a normal partition-based filesystem.
vgmknodes Remake the device files of VGs
These first few steps do not yet display any
vgsplit Split one VG into two
particular strengths since all we have done is create a
e2fsadm Change size of LV and Ext2 file system
virtual partition on a physical one. If the LV becomes
lvmchange Reset LVM
too small and there is still free capacity in the VG, we
lvmsadc Collect statistical data
can expand it without re-installation or a reboot. This
lvmsar Display collected statistical data
is done with the command
lvmcreate_initrd Create initial RAM disk to boot with root file system on LV
lvmdiskscan Scan for devices supported as PV
lvextend -L+200M /dev/mevg/melv
which adds a further 200Mb to the 100Mb. Since the
commands. To get you started, there is a basic filesystem stored in the LV is not (yet) automatically
introduction with a list of all the commands (man expanded at the same time, a filesystem command
lvm). has to take over this task. If one uses Ted Ts o s
resize2fs, the filesystem must not be mounted; on
Fdisk indispensable the other hand, Andreas Dilger s ext2online is capable
To avoid unintentionally overwriting a partition of expanding Ext2 filesystems in mounted condition
already in use with pvcreate, partitions must be set providing you have the necessary kernel patch for
via fdisk to the type reserved for LVM, 0x8E; only this:
then can pvcreate be used on them. It is in any case
resize2fs /dev/mevg/melv
advisable to create at least one partition, even if the
whole disk is to be used as PV under LVM. The
advantage is that this then appears under /proc/ Both tools are supported by the e2fsadm program,
partitions and is displayed in fdisk, so it simply cannot which is supplied with Linux LVM, which executes
appear unused later by mistake, if one invokes fdisk - lvextend and resize2fs in Ext2 resizings.
l, for example. The disadvantage that the partitions
e2fsadm -L+200M /dev/mevg/melv
table (one sector) for the PV gets lost is an
acceptable price to pay.
Figure 3 shows the main inputs and outputs for the
Simple practical examples
If you have created /dev/ sde1 as described and have
set the type to 0x8E, you can use
pvcreate /dev/sde1
to create a first PV, then with
vgcreate mevg /dev/sde1
a first VG named mevg (my first VG). If this is
successful, vgcreate automatically loads the necessary
metadata in the LVM driver, so that subsequently the
mapping tables of existing LVs are available or tables
of newly created LVs can be loaded. Seen another
way, vgcreate creates our first virtual disk, which (still)
contains a physical disk partition, and activates the
VG for further use.
The first LV is created with the command Figure 3: Installing and expanding a filesystem
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LINUX MAGAZINE Issue 16 " 2002
KNOW HOW
Info
Alessandro Rubini & Jonathon Corbet Linux
Device Drivers (O Reilly)
LVM HOWTOs http://tech.sistina.com/lvm/
doc/lvm_howto
LVM homepage http://www.sistina.com/
products_lvm.htm
relocated to PEs of other PVs without data loss, with
the aid of pvmove. After that, the free PV can be
Figure 4: Instead of calling up lvextend and resize2fs
separately, one can also use e2fsadm
removed from the VG with
vgreduce mevg /dev/sde1
little sample session. The last two commands
(lvextend and resize2fs) can be replaced by calling up
e2fsadm (see Figure 4). to add it to another VG whose capacity has now
Since expanding or newly creating LVs can easily become too small, for example.
make our VG reach the limits of its capacity, it s Another instance when pvmove may be used is to
possible to add additional disk space after replace partition A on a disk, which is too small or
installation. New PVs are created as described above. too slow (/dev/ sde) by a larger or faster one (B on
If /dev/ sdb1 is available as an additional partition, the /dev/ sdb). Provided there is a free connection for B,
new PV is initialised via one would first add this to the VG, in order to then
move all data with
pvcreate /dev/sdb1
pvmove /dev/sde1 /dev/sdb1
The VG mevg is then expanded using:
from A to B. If only A and B are contained in the VG, it
vgextend mevg /dev/sdb1
is superfluous to specify the device file of B (/dev/ sdb1),
since apart from this, no other destination PV exists.
Then the extra storage space is immediately available Figure 5 shows the expansion and reduction of the VG
for the creation or expansion of LVs. If it is not together with the relocation of data with pvmove.
possible to install additional hard drives in advance, You ve been introduced to some of the standard
all the steps for expanding the VG can be done applications of the Linux Logical Volume Manager.
without rebooting. Owners of Hot-Plug SCSI do not For more in-depth information, the LVM-Howtos and
even have to reboot after installing new SCSI disks. the general guide over at http://www.sistina.com are
highly recommended.
Removing disks
If you want to remove PVs from a VG, these must be
Figure 5: pvmove
relocates data onto
empty. In other words, none of their PEs must be
other disks
assigned to any LVs. This can be checked using the
instruction:
pvdisplay -v /dev/sde1
The mapping of a specific LV can be found with:
The author
lvdisplay -v /dev/mevg/melv
Heinz Mauelshagen is the
author of the Logical
If PEs are occupied, but sufficient capacity is free on Volume Manager for
other PVs of the same VG, the PV can be emptied Linux. He works at Sistina
with Software, Inc., which
specialises in file system
pvmove /dev/sde1
development; in addition
to LVM, they also
(the option -v displays the relocation of the maintain GFS.
individual LEs). Data in assigned PEs can be
59
Issue 16 " 2002 LINUX MAGAZINE
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