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AIX Logical Volume Manager

News	See also	Redbooks	Recommended Links	LVM Tasks	Command Summary	Reference
what disks are in the root volume group	How many disks are on the system	Create a volume group	Add a disk to the root volume group	Make sure the disks are in an active state	Mirror the logical volumes	Create the boot image on the new disk
JFS	mksysb Command	Aix JFS2 snapshots	Tips	History	Humor	Etc

LVM stands for Logical Volume Manager and JFS is a Journaled File System.

The role of LVM is to present a simple logical view of underlying physical storage space, ie. harddrive(s). LVM manages individual physical disks - or to be more precise, the individual partitions present on them (for a short glossary of terms, look at the end of the article). LVM hides the numbers, size and location of physical partitions from users. Instead it presents the concept of logical volume. A logical volume may correspond to a physical partition (but that obviously almost defeats the purpose of LVM) but it doesn't have to. One volume may be composed of several partitions located on multiple physical disks. Not only that, the volumes can even be extended (not shrunk - people usually want more space, not less). They can even be extended while the OS is running and the filesystem is being accessed! Of course, most home and SOHO users don't have the hardware required for this.

The more experienced readers are now probably wondering how 'traditional' file systems like FAT or HPFS could be extended at runtime. The answer is, they can't. To take full advantage of LVM, it is necessary to use a filesystem designed for it. JFS is not really tied to LVM, both LVM and JFS can exist separately, but only when working in concert both can reach their full potential.

Physical View of the LVM

The following is a list of the common features in the LVM and VxVM methods of storage management:

Online backup of file systems
Snapshot backup of file systems
Quick resynchronization of changed partitions after online or snapshotbackup
Migration of data with active volumes
Transparent data stream switch after mirrored disks fail
Ability and limits to operate in a multinode concurrent configuration
Commands to replace dead or failing drives
Hot spare, standby disks

Both VxVM and LVM enable file systems to be expanded while the file system is mounted and in use. Both volume managers are tied closely to specific file systems. VxVM only works with the Veritas File System, and LVM only works with the JFS or JFS2 (enhanced JFS) file system found on AIX. The one difference between VxVM and LVM is the philosophy on the concept of file system shrinkage. LVM/JFS/JFS2 does not allow the user the ability to shrink the file system. The reasoning behind this is that sometimes portions of files are spread out over the file system for performance reasons. And when a file system needs to be shrunk, the file system must go out and gather up the "pieces" of a file and relocate them to a portion of the file system that will not be eliminated. This method is time-consuming and not very efficient for performance. Thus on AIX, file system reduction is not permitted on JFS and JFS2.

VxVM takes a different approach to this. It does allow the reduction of a file system size, but it is very simplistic and has drawbacks. They literally "chop" off the end of the logical volume and file system up to the point where the user wants the file system reduced. Warnings are included in the VxVM product regarding this practice; if the user has data at the end of the file system, the data may be corrupted or lost. Thus, for the purposes of this paper, the ability of VxVM to reduce file system size is considered limited.

Terminology

Partition - a portion of physical hard disk space. A hard disk may contain one or more partitions. Partitions are defined by PC BIOS and described by partition tables stored on a harddrive. Every PC OS understands partitions.
Volume - a logical concept which hides the physical organization of storage space. A compatibility volume directly corresponds to a partition while LVM volume may span more than one partition on one or more physical disks. A volume is seen by users as a single drive letter. Only WSeB and eCS understand LVM volumes.
DASD - Direct Access Storage Device. A term often used by IBM instead of 'hard disk' to confuse mere mortals.
Physical Volume (PV) Synonym for "hard disk". A single physical hard drive.
Volume Group (VG) A set of one or more PVs which form a single storage pool. You can define multiple VGs on each AIX system.
Physical Partition (PP) The smallest allocation unit in the LVM. All PPs within a VG are the same size, usually 4 or 8 MB.
Volume Group (VG) A set of one or more PVs which form a single storage pool. You can define multiple VGs on each AIX system.
Logical Partition (LP) A logical mapping to one or more PPs within the same VG, regardless of whether they're on the same PV or not.
Logical Volume (LV) A set of one or more LPs within the same VG which form a usable unit of disk space. LVs are used analogously to partitions on PCs or slices under Solaris: they usually contain filesystems or paging spaces ("swap").
Rootvg Default volume group created during installation. The vg holds the OS filesystems ( /,/usr, /home, /proc /opt, /tmp, /var and swap space )

Big Picture View of the LVM

Advantages:

Filesystems can span disks, so size is not limited by capacity of physical disks.
Can expand filesystems on the fly.
Can add additional disks to existing pool of disk space (VG).
Can mirror important data on multiple physical disks for redundancy.
Can "export" an entire VG so that all disks in the VG can be easily physically disconnected, moved to another machine, and "imported".

Limitations:

Must reducevg when removing disk.
When a single disk in a VG dies, the whole VG is affected.
"Brick Wall" between VGs - LVs can't cross VG boundary.
Cannot shrink filesystems.

General Approach:

Configuring for performance or flexibility vs. reliability or fault-tolerance.
Strategy: Define adequate but not huge filesystems, leaving pool of free storage for later assignment.
Seperate out data categorically onto different VGs, so that an OS upgrade will not touch user data, or so that user data can be easily moved to another machine.

Filesystems

/etc/filesystems - AIX's version of /etc/fstab. Uses stanza format, * is comment character.
Useful commands:
- crfs - Creates logical volume, makes filesystem on logical volume, adds entry to /etc/filesystems. Does not mount the new filesystem for you!
- chfs - Changes settings of filesystem, logical volume, or /etc/filesystems entry.
- rmfs - Removes filesystem, logical volume, entry from /etc/filesystems, and optionally the mount point.
AIX itself is always installed in a VG called rootvg
The jfs filesystem is a journaling filesystem, meaning that it keeps a summary of filesystem update information on a special LV of type jfslog

Command Summary

Command	Definition
chfs -a size=<#512 byte blocks> <file system>	Increases the size of a journaled file system to the total number of 512 byte blocks specified
chfs -a size=<+512 byte blocks> <mount point>	Increases the size of a journaled file system by the addional number of 512 byte blocks specified. For example "chfs -a size=+393216 /usr"
chlv -n <newname> <oldname>	Change the name of a logical volume (it must be inactive)
crfs -v jfs -m <mount point> -g <volume group> -a size=<# of 512 byte blocks> crfs -v jfs -m <mount point> -d <logical volume>	This command makes a logical volume, mount point with a journaled file system: creates a jfs file system on a logical volume
df -k	Shows the disk usage of logical volumes on the server.
exportvg <volume group>	removes a volume group from a machine
extendvg <volume group> <physical volume>	Adds a new physical volume to an existing volume group
importvg -y <volume group> <physical volume>	add a volume group to another machine
lslv <logical volume> [-l, m]	Lists information about the logical volumes. The -l option lists the disks in the logical volume.
lspv <physical volume> [-l, M, p]	Lists the disks on the server, including the physical volume will give details about that disk. The -l option will list the details of how the filesystems are distributed on the disk.
lsvg <volume group> [-l]	Lists the volume groups on the server, including the volume group name will give details about that vg. The -l option will list the logical volumes in the volume group.
lsvpcfg	Lists each vpath and the hdisks that make up the vpath
mklv -y <new lv> <vg>	Makes a logical volume in a volume group
mksysb -l -f <device>	makes a bootable backup of rootvg
mkvg -y <volume group> <physical volume> . . . <physical volume>	Makes a volume group out of one or more physical volumes
mount <logical volume> <file system> or mount <filesystem> if it is already in /etc/filesystems	Mounts the file system for use.
reducevg <volume group> <physical volume>	Removes a physical volume from a volume group
rmfs <file system>	removes a file system and it's logical volume
rmlv <lv>	Removes a logical volume (it must be inactive)
savevg -l -f <device> <volume group>	makes a backup copy of another volume group
umount <file system> dismount the file system	Unmounts the filesystem.

Example: Adding a Disk

Physically attach the disk to the system. Make sure there are no SCSI ID conflicts.
Run cfgmgr to detect new device. Device will be installed as hdiskn, where n is some integer.
Run extendvg vgname hdiskn to add the disk to a VG named vgname.
To create a new filesystem on this VG, use crfs -v jfs -g vgname -A yes -a size=262144. (Note that the size parameter indicates the number of 512-byte blocks, so 262144 means 128MB.)
To add 128MB to an existing filesystem on this VG, use chfs -a size=+262144 /filesystem/name.

Filesystem Procedures

Procedure to create a filesystem using JFS:

See below the procedure for creating a logical volume and a filesystem using JFS:

Procedure to extend the size of filesystem using JFS:

"df" to see the filesystem, it's current size, % utilization and the name of it's logical volume
"lslv <logical_volume>" to show information about the logical volume including it's volume group name.
"lsvg <volume_group>" to show information about the volume group, including number of free pp's and the pp size
If there are not enough free pp's then see below for procedure to add a disk to a volume group.
"chfs -a size= +4194304 <MOUNT_POINT>" to grow the filesystem by 2 GB (4194304=2*1024*1024*1024/512)
- NOTE: Growing the file system will automatically grow the logical volume
df" shows the file system's current size is 2 GB more than before.

Troubleshooting extending the size of a filesystem using JFS:

Error Message: 0516-787 extendlv: Maximum allocation for logical volume <LV_Name> is 512.
- Maximum number of LPs for the logical volume has been exceeded - must increase the allocation
- Calculate the number of LPs needed = LV Size in MB / LP size in MB
- chlv -x <new_max_lps> <logical_volume>

Procedure to remove a file system

Unmount the filesystem
Remove the logical volume "rmlv <lv_name>"
Remove the filesystem information from /etc/filesystems

Procedure to reduce the size of a file system - shareold is 8mb and needs to be reduced to 4mb

Create the file system
1. crfs -v jfs -m /usr/sharenew -g rootvg -a size=8192
2. this makes a logical volume in the root volume group of 4MB that uses jfs
Mount the volume
1. mount /usr/sharenew
Move the files from the old file system (/usr/shareold)
1. cd /usr/shareold
2. tar cf - | (cd /usr/sharenew; tar xvf -)
3. cd
Unmount the file systems
1. umount /usr/sharenew
2. umount /usr/shareold
Remove the old file system and it's logical volume
1. rmfs /usr/shareold
1. chfs -m /usr/shareold /usr/sharenew
Mount the new filesystem
1. mount /usr/shareold
Delete the temporary mount point
1. rmdir /usr/share

Logical Volume Procedures

Procedure to create a logical volume and filesystem in a volume group using JFS:

lsvg to determine the size of the PP
lslv in similar logical volumes to determine if mirroring is in effect
Calculate the number of PPs needed for the logical volume
1. bc
2. scale=2
3. <size of lv in MB>/<size of PP in MB>
4. quit
mklv -y "<LV_NAME>" <VG_NAME> <# of LPS> --> creates the logical volume
crfs -v jfs -d <LV_NAME> -m /<MOUNTPOINT> -A yes --> makes the filesystem, creates the mountpoint and puts it in /etc/filesystems
mount /<MOUNTPOINT> --> mounts the new fileystem
df /<MOUNTPOINT> --> verifies the mount and the size of the new filesystem
Check the ownership and permissions of the new mount point
- ls -ld <mountpoint>
- chown owner:group <mountpoint>
- chmod XXX <mountpoint>
If mirroring is in effect, then mirror this logical volume to another disk (original and 1 mirror):
- mklvcopy -s y <LV_NAME> 2

Check to see if all of the logical volumes in a volume group are mirrored

lsvg -l

Mirror a logical volume after the fact

mklvcopy -s y <LV_NAME> 2

Volume Group Procedures

Procedure to create a volume group:

lsdev -C -c disk -> lists available disks (and the hdisk#) on the server
mkvg -y "<VG_NAME>" hdisk# --> creates the volume group on the named hard disk
varyonvg <VG_NAME> --> activates the volume group

Procedure to add a disk to a volume group (extend the volume group)

extendvg <vg> <disk#>
- Verify the disk has been successfully added to the vg
lsvg -p <vg>

Procedure to mirror the rootvg:

lspv --> determine the hdisk#
extendvg rootvg hdisk<number> --> add the hdisk to the volume group
lspv --> verify that the hdisk has been successfully added to the volume group
chvg -Q 'n' rootvg --> change the quorum so that the vg will stay active if one of the mirrors fail
mirrorvg -S -c 2 rootvg --> mirror all of the logical volumes in the volume group
lsvg -l rootvg --> verify successful mirroring (pps will appear "stale" until synchronization is complete).
bosboot -a --> update the boot image information
bootlist -m normal -o hdisk0 hdisk1 --> create a new bootlist
bootlist -m normal -o --> verify the bootlist is correct

Procedure to increase the number of LP's available
Assume we receive an error that the maximum number of LP's had been exceeded, and the maximum number of LP's defined was 1100:

"lsvg <volume_group>" to show the total PP's available in the volume group =1250
"lsvg -l <volume_group>" to show the total PP's used in all logical volumes in that volume group (showed sys1log, the jfs log was using 2 PP's)
"chlv -x 1248 <logical_volume>" to change the maximum number of LP's from 1100 to 1248 (1250 PP's in the volume group - 2 PP's used by the jfs log = 1248 available)

Physical Disk Procedures

Procedure to find disks/vpaths that are unallocated

lsvpcfg
- This will show disks/vpaths and the volume group they are allocated to
lspv|grep None
- This will show pvs and whether they are asssociated with a volume group
- Note: For vpaths, the hdisks will show as none, but they may be allocated to a vpath - you must grep each hdisk with the lsvpcfg

Procedure to make a new lun available to AIX

Allocate the new lun on the SAN
Run "cfgmgr"
Verify the new vpatch/hdisk by running "lsvpcfg"
- There should be a new vpath and it should be available with no volume group - if not, rerun cfgmgr

Procedure to list the PVs in a volume group:

lsvg -p <volume group>

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NEWS CONTENTS

20090420 : AIX Support Tips ( AIX Support Tips, Apr 20, 2009 )
20090415 : Logical Volume Manager Basics ( Logical Volume Manager Basics, Apr 15, 2009 )
20090414 : IBM developerWorks Wikis - AIX - LVM ( IBM developerWorks Wikis - AIX - LVM, Apr 14, 2009 )

Old News ;-)

[Apr 20, 2009] AIX Support Tips

File system - allocate more space

chfs -a size='+500' /usr/acct (To enlarge file system)

Use chlv -x 256 <lv-name> if you get an errorwhen running the above command (To expand beyond maximum LV's)

[Apr 15, 2009] [PDF] Logical Volume Manager Basics

Good SHARE presentation

[Apr 14, 2009] IBM developerWorks Wikis - AIX - LVM

Logical Volumes striping

From AIX 4.3.3 (not backward compatible) LVM support striping

Often called fine striping (i.e. not PP level striping)

Recommend 32K, 64K or 128K stripe width = 4 to 16 times I/O size most databases do 4K or 8K I/O

Excellent for balancing serial and random I/O across disks

Excellent read ahead serial I/O boost

Using LVM commands (with smit)

Find unused disks: lspv

Add them to a VG or create a VG: smitty lvm + VG + Add VG

Check unused space in VG: lsvg <vgname>

Create a LV: smitty lvm + LV + Add LV

Create a Filesystem (JFS)smitty jfs + Add jfs + standard + which VG (see below)

If the command changes the VG, PV or LV then the VG is locked i.e. one command at a time.
This is needed to allow recovery of the change - if the system fails during the update.

Creating Stripes Tips:

Stripes LV must contain a whole number of items, that is the number of PPs must divide evenly into the number of disks

Or put another way: each disk have the same number of PPs

Hint: use multiples of No. of disks to make this easier

LVM Create Mirrors

There are three ways to create a mirror:

One Step = create LV with mirror

These can be slow as you can have to wait until mirror complete before creating next lv

Create LV and later add mirror synchronise=yes

again slow as you can have to wait for silvering
#Create LV and later add mirror synchronise=no and then sychronise all mirrors together

Use this when creating lots of lv because all mirrors can be created at the same time

So (with smit)

create lv's

add mirrors sync=no

syncvg -l <lvname>
to remove "stale" partitions = resilver
Warning:

resilvering hurts performance

often forgotten when recommending mirror breaking for backup purposes

LVM Higher Level Management Commands

Disown disks ready to connect them to another system

varyoffvg <vgname>

exportvg <vgname>
or smitty lvm

Activate newly attached disks

cfgmgr – finds disks

importvg hdiskN – any one of the disks

varyonvg <vgname> – mounts file systems

Moving LVs

migratelv (not striped)

Use smit lvm

Can migrate LV on a disk to another disk

Can empty whole disk to remove it later or replac a fault disk
Disks can be added and removed from a VG

extendvg and reducevg

Use smit lvm recommended

LVM rootvg good practice

Small rootvg - 1 or 2 disks (for mksysb backup)
Mirror rootvg for AIX disk protection

Add disk hdisk1

chvg -Qn rootvg

For hd1 to hd8 and hd9var copy the LV

mklvcopy hd{1,2,3,4,5,6,7,8,9var} 2 hdisk1

syncvg -v rootvg

bosboot -a

bootlist -m normal hdisk0 hdisk1
Or

mirrorvg -s rootvg

this second option is recommended

TIP: This command will show you formatted list of active Volume groups
for i in $(lsvg -o);do lsvg $i;done \
| awk ' BEGIN { printf("%-10s\t%10s\t%10s\t%10s\t%10s\n","VG","Total(MB)","Free","USED","Disks") };/VOLUME GROUP:/ \
{ printf("%-10s\t", $3) };/TOTAL PP/ { B=index($0,"(") + 1;E=index($0," megaby");D=E-B;printf("%10s\t", substr($0,B,D) );};/FREE PP/ \
{ B=index($0,"(") + 1;E=index($0," megaby");D=E-B;printf("%10s\t", substr($0,B,D) );};/USED PP/ \
{ B=index($0,"(") + 1;E=index($0," megaby");D=E-B;printf("%10s\t", substr($0,B,D) );};/ACTIVE PV/ { printf("%10s\t\n", $3) } '
 
Put other disks into other volume groups

use min PP size ( 8MB or 16 MB)

use 16 to 32 disks per volume group

use different vg for data that might be moved elsewhere
Avoid hot disks

Spread data across disk with LVs or RAID5

Mirrors add protection and performance

Use striping whenever possible

Think spindles not disk size
reorgvg

this will move physical partitions (PP) around in an attempt to get PPs in the right place
migratepv

this can be used to spread PPs across move disks

also to remove all PPs from a suspect disk before replacement (due to errors being reported)

AVOIDING Disk Crashes

Rule One: RAID5 or mirror everything

Rule Two: monitor error logs. Make sure you know when a disk failed

Rule Three: call hardware support. That is what they are for

Rule Four: Don't meddle. Only try, if you know what you are doing

Rule Five: Read and practise. Get the Redbooks and try it safely

Reference

http://www.redbooks.ibm.com/redbooks/SG246584.html

Chapter 6. Logical Volume Manager and disk management 129 Naming conventions for LVM Use Table 6-5 to find out the names that LVM uses for each of its components. Table 6-5 Naming conventions for LVM 6.3 Working with logical volume manager In this section, we will describe the way to list, create, remove, and change characteristics for volume groups, logical volumes, and physical disks. We mainly use VxVM and LVM, because not all the features in these two products are available in Solaris DiskSuite. 6.3.1 Volume groups As defined earlier, a volume group is a collection of physical disks that are related. Let us review the main and most important ways to work with them. Listing a volume group: LVM In AIX 5L Version 5.1, we use lsvg to list a volume group. If you do not specify an option, it will show you all the volume groups defined in the system. Some of its useful flags are: -o Shows only the active volume groups. -p <vg_name> Shows all the physical volumes that belong to the requested volume group (vg_name).

Chapter 6. Logical Volume Manager and disk management

157 r Removes all disks from the pool of hotspare disks for the volume group specified. Synchronization policy Since AIX 5L Version 5.1, there is another option (-s) for the chvg command. It is used to specify the synchronization characteristics. The supported values for this option are: y Automatically attempts to synchronize stale partitions. n Will not attempt to synchronize stale partitions. This is the default value. Here we have some examples for hot spares: # chpv -hy hdisk2 This command marks hdisk2 as a hotspare disk for the volume group to which it belongs. To change the synchronization policy, we use chvg, as shown in the following example. It is highly recommended that you change the synchronization policy to automatic. # chvg -hy -sy apachevg 6.4 Quick reference In both LVM and VxVM, the same task can be done in different ways: AIX LVM tools smitty, Web-based System Manager (GUI), or the command line VxVM 3.2 Tools /opt/VRTSvmsa/bin/vmsa (GUI), vxdiskadm (text based interactive tool), or the command line Table 6-6 on page 158 contains a quick reference for the most used tasks, using command line tools.