The problem: Downsizing a physical volume
The IBM AIX LVM has several features that
allow you to reclaim unused disk space without downtime. You can reduce a file
chfs, and you can remove unused physical
volumes (PVs) from volume groups (VGs) so that you can allocate the storage
However, if you want to reduce the size of an AIX PV to reclaim unused disk space, you will damage the data on the PV. If you have a large SAN LUN that has a significant amount of unused physical partitions (PPs), you could back up the data, reduce the LUN size, and restore the data to the new, smaller PV. But that could involve unacceptable downtime. If, after a data cleanup, a large LUN needs to have some of its space reclaimed, it should be as seamless a procedure as possible.
The solution: Migrate to a new, smaller physical volume
It may not be possible to downsize a PV that is in use, but you can create a new, smaller
SAN LUN, add it to the existing VG, and then migrate data from the larger PV to
the smaller one. When that is done, the original oversized PV can be removed from
the VG. The hdisk can then be taken out of the Object Data Manager (ODM) using
rmdev. Then, you can recycle the SAN storage for some
This procedure requires the new PV to have a suitable size and characteristics for adding
to the existing VG. It must be large enough to hold all the data that is on the original
PV (the used PPs). The procedure also assumes that there is no logical volume (LV)
striping that would restrict the ability to run a mirror of a logical volume using
For this exercise, there is a VG called datavg with a 50 GB PV. The
lspv command shows the total size of the PV as well as the free
and used PPs (see Listing 1).
Listing 1. Displaying physical volume characteristics
# lspv hdisk1 PHYSICAL VOLUME: hdisk1 VOLUME GROUP: datavg PV IDENTIFIER: 00cb07a45a12b4ca VG IDENTIFIER 00cb07a400004c00000001345a26db3e PV STATE: active STALE PARTITIONS: 0 ALLOCATABLE: yes PP SIZE: 512 megabyte(s) LOGICAL VOLUMES: 1 TOTAL PPs: 99 (50688 megabytes) VG DESCRIPTORS: 2 FREE PPs: 0 (0 megabytes) HOT SPARE: no USED PPs: 99 (50688 megabytes) MAX REQUEST: 256 kilobytes FREE DISTRIBUTION: 00..00..00..00..00 USED DISTRIBUTION: 20..20..19..20..20 MIRROR POOL: None
There is a single enhanced journaled file system (JFS2) called /scratch that has more than 35 GB free of its 49.50 GB allocation. This file system was created with an INLINE JFS2 log:
# df -gI /scratch Filesystem GB blocks Used Free %Used Mounted on /dev/scratchlv 49.50 14.20 35.30 29% /scratch
Reduce the file system
Because the file system is using less than one third of its allocation, its total
disk allocation can be reduced. You can reduce the file system size using
chfs. The following command reduces it by 30 GB:
# chfs -a size=-30G /scratch Filesystem size changed to 40894464 Inlinelog size changed to 78 MB.
The total disk allocation for the file system has been reduced to 19.50 GB:
# df -gI /scratch Filesystem GB blocks Used Free %Used Mounted on /dev/scratchlv 19.50 14.08 5.42 73% /scratch
This process has freed up some PPs on the PV, as the
command in Listing 2 shows.
Listing 2. lspv showing free physical partitions
# lspv hdisk1 PHYSICAL VOLUME: hdisk1 VOLUME GROUP: datavg PV IDENTIFIER: 00cb07a45a12b4ca VG IDENTIFIER 00cb07a400004c00000001345a26db3e PV STATE: active STALE PARTITIONS: 0 ALLOCATABLE: yes PP SIZE: 512 megabyte(s) LOGICAL VOLUMES: 1 TOTAL PPs: 99 (50688 megabytes) VG DESCRIPTORS: 2 FREE PPs: 60 (30720 megabytes) HOT SPARE: no USED PPs: 39 (19968 megabytes) MAX REQUEST: 256 kilobytes FREE DISTRIBUTION: 00..01..19..20..20 USED DISTRIBUTION: 20..19..00..00..00 MIRROR POOL: None
Add a smaller physical volume to the volume group
The next step is to add a new, smaller PV to the existing VG. This PV should have at least the same redundancy and input/output (I/O) performance as the original, larger PV. For example, it should be of an equivalent Redundant Array of Independent Disks (RAID) array. Any other tuning characteristics—such as queue depth— should be set to ensure that the system performance is comparable to the original, larger PV.
Create a new LUN, and allocate it to the AIX logical partition (LPAR). In this example, the new LUN is 20 GB:
The output of the
lspv command shows that the new disk is called hdisk2
(see Listing 3) and it does not yet belong to a VG.
Listing 3. Listing the new disk
# lspv hdisk0 00c5a47e3f356f3c rootvg active hdisk1 00cb07a45a12b4ca datavg active hdisk2 none None
You can view the size of the disk even before it is added to a VG using the
getconf command. This reports the size in MB:
# getconf DISK_SIZE /dev/hdisk2 20480
Add the disk to the existing VG using the
# extendvg datavg hdisk2 0516-1254 extendvg: Changing the PVID in the ODM.
Mirror or migrate the logical partitions to a new physical volume
You can mirror the LV to the new PV, and then remove the copy from the original PV after all the PPs have been synchronised between the two PVs.
mklvcopy -k scratchlv 2
There are other options for
mklvcopy. For example, you
can postpone the synchronization until a quieter time. You can also specify the
disk allocation policy. The official documentation for
has the necessary details (refer to Resources).
Remove the copy from the original physical volume
When the synchronization is complete, you can remove the copy from the original
rmlvcopy. Be sure to specify the PV that has
the copy you want to remove.
rmlvcopy scratchlv 1 hdisk1
You can use the
lspv command to confirm that there
are no more used PPs on the original PV. If there are still some PPs in use, you
can list the LVs on the PV using
When you are sure that all the PPs have been moved to other PVs, the original PV can
be removed from the VG using
reducevg datavg hdisk1
You should be able to remove the original larger PV from the ODM using
rmdev -d -l hdisk1
Finally, you can remove the LUN or allocate it for some other use.
Cutting out outages
As you can see, the LVM features allow you to move data around—even to smaller disks—without unnecessary user impact. By taking advantage of the LVM mirroring and migration capabilities, you can keep your system up and still recover the much-needed storage space if it has been over-allocated.
- The LVM can simplify moving to a new SAN storage. This approach is outlined in Using the AIX Logical Volume Manager to perform SAN storage migrations (Chris Gibson, developerWorks, July 2010).
- For details on the LVM commands referred in this article, refer to the AIX Information Center.
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- Technology bookstore: Browse the technology bookstore for books on this and other technical topics.
- The article Live Transplant (Anthony English, IBM Systems Magazine, April 2011) shows how the AIX LVM helps you to migrate data without downtime.
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