AIX: LVM Overview

LVM Theory

Physical Volume = PV is IBM speak for a disk (it could be worse some IBMers still refer to DASD! (Dyanical Access Storage Device which is a main frame term). They are:

• Named by AIX as hdisk0, hdisk1, hdisk2, …
• Regardless of the underlying technology (SCSI, SSA sort of IBM’s early SAN, SAN, RAID5 using the adapter)
• Disk and AIX automatic bad block reallocation

Volume Group = VG is IBM speak for a group of disks
Volume Group operations:

• Disk space always allocated within single VG
• All disks available in AIX or none – work as a group
• Can be exported to be attached to other AIX – allows High Availability HACMP
• First VG called rootvg

Root Volume Group (rootvg) is created automatically while installing AIX is placed within this VG

• AIX files
• Initial paging
• Usually only the first disk
• Or two, to allow mirroring of rootvg
• Often internal disks
• Recommend: Keep to a small number of disks

Other Volume Groups

• Other VG created by the System Admin.
• Name typically related to their use
• Optionally ending in “vg”

Volume Groups

• One or more disks
• Can later add a disk to a VG
• Can remove disk from VG – if empty
• Disks within VG cut into chunks called
• Physical Partitions (PP)
• Minimum allocation unit
• Typically, 8, 16 or 32 MB (older 2GB disks = 4 MB)
• Nothing smaller can be allocated
• Each VG fixed PP size.
• Volume group has total, free and used PPs                                                                               *Disks do not need to be the same size or even the same technology

Logical Volumes

• It is a large piece of disk
• Can be used for lots of tasks like
• raw device or file system or paging space
• Created within single VG
• LV has many attributes like       

                can be one disk or more disks or all disks of VG 
                mirrored or striped
                particular part of disk
                Sizes is in Physical Partitions – example: id PP = 16 MB and LV size of 64 PPs = GBPPs allocated by policies

• min or max spread across disks
• min=fill disk then move on, max=evenly spread
• disk placement centre, middle, edge
• mirrors via extra copies
• stripes via stripe width
• MWC (Mirrored write consistancy) and others

Logical Volumes Admin

• Can be grown in size
• Can be removed
• Can be moved to different disks within VG
• Can later have a filesystem created in it
• Attributes can be changed like: mirror added or removed or policy changed

LVM in practice

The disks can be apprached from three different directions and from there you can see the relationship to the others.
As a reminder

• pv = physical volumes – real disk or LUN
• lv = logical volume
• vg volume group

To the this use the ls commands:

• lspv
• lslv
• lsvg

In more detail

Lists volume group names: lsvg

• List VG details: lsvg
• List LVs within VG: lsvg -l
• List PV within VG: lsvg -p

Lists physical group names: lspv

• List PV details: lspv hdiskN
• List LVs within PV: lspv -l hdiskN
• List PP within PV: lspv -p hdiskN
• Lists LV names: lsvg -l
• List LV details: lslv
• List LVs within PV: lslv -l
• List PP within PV: lslv -p
• Show mirrored LV: lslv -m

When you create a Logical Volume with smitty you will see a panel like this:

smitty create logical partition

Add a Logical Volume
Logical volume NAME [scratch]
VOLUME GROUP name testvg
Number of LOGICAL PARTITIONS [64]
# PHYSICAL VOLUME names [hdisk1 hdisk2 hdisk3] +
Logical volume TYPE [jfs]
POSITION on physical volume middle +
RANGE of physical volumes minimum +
MAXIMUM NUMBER of PHYSICAL VOLUMES [] # to use for allocation
Number of COPIES of each logical partition 2 +
partition Mirror Write Consistency? yes +
Allocate each logical partition copy on a SEPARATE physical volume? yes +
RELOCATE the logical volume during reorganization? yes +
Logical volume LABEL []
MAXIMUM NUMBER of LOGICAL PARTITIONS [512]
# Enable BAD BLOCK relocation? yes +
SCHEDULING POLICY for reading/writing logical partition copies parallel +
Enable WRITE VERIFY? no +

Some of the terms used are described below:

Mirrors

On the disk placement

Mirror Scheduling Policy

Mirror Write Consistency

If you are terribly and extremely unlucky … a crash could happen leaving the two mirrored copies of a block different.
Reading the block could give you the old or new block.
With a RDBMS, it does not mind disk is newer but must be consist when read.
MWC reminds LVM to “resilver” these recently written blocks in recovery – to make them the same.

Physical Partition (PP) Allocation

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
• 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                                                                                                                                          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
• exportvg
• or smitty lvm

Activate newly attached disks

• cfgmgr – finds disks
• importvg hdiskN – any one of the disks
• varyonvg – 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(“%-10st%10st%10st%10st%10sn”,”VG”,”Total(MB)”,”Free”,”USED”,”Disks”) };/VOLUME GROUP:/ { printf(“%-10st”, $3) };/TOTAL PP/ { B=index($0,”(“) + 1;E=index($0,” megaby”);D=E-B;printf(“%10st”, substr($0,B,D) );};/FREE PP/ { B=index($0,”(“) + 1;E=index($0,” megaby”);D=E-B;printf(“%10st”, substr($0,B,D) );};/USED PP/ { B=index($0,”(“) + 1;E=index($0,” megaby”);D=E-B;printf(“%10st”, substr($0,B,D) );};/ACTIVE PV/ { printf(“%10stn”, $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: don’t let this stop your system
• 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