To create a LVM, we follow a three step process.
Step One : We need to select the physical storage resources that are going to be used for LVM. Typically, these are standard partitions but can also be Linux software RAID volumes that we've created. In LVM terminology, these storage resources are called "physical volumes" (eg: /dev/hda1, /dev/hda2 ... etc).
Our first step in setting up LVM involves properly initializing these partitions so that they can be recognized by the LVM system. This involves setting the correct partition type (usually using the fdisk command, and entering the type of partition as 'Linux LVM' - 0x8e ) if we're adding a physical partition; and then running the pvcreate command.
# pvcreate /dev/hda1 /dev/hda2 /dev/hda3
# pvscan
The above step creates a physical volume from 3 partitions which I want to initialize for inclusion in a volume group.
Step Two : Creating a volume group. You can think of a volume group as a pool of storage that consists of one or more physical volumes. While LVM is running, we can add physical volumes to the volume group or even remove them.
First initialize the /etc/lvmtab and /etc/lvmtab.d files by running the following command:
# vgscan
Now you can create a volume group and assign one or more physical volumes to the volume group.
# vgcreate my_vol_grp /dev/hda1 /dev/hda2
Behind the scenes, the LVM system allocates storage in equal-sized "chunks", called extents. We can specify the particular extent size to use at volume group creation time. The size of an extent defaults to 4Mb, which is perfect for most uses.You can use the -s flag to change the size of the extent. The extent affects the minimum size of changes which can be made to a logical volume in the volume group, and the maximum size of logical and physical volumes in the volume group. A logical volume can contain at most 65534 extents, so the default extent size (4 MB) limits the volume to about 256 GB; a size of 1 TB would require extents of atleast 16 MB. So to accomodate a 1 TB size, the above command can be rewriten as :
# vgcreate -s 16M my_vol_grp /dev/hda1 /dev/hda2
You can check the result of your work at this stage by entering the command:
# vgdisplay
This command displays the total physical extends in a volume group, size of each extent, the allocated size and so on.
Step Three : This step involves the creation of one or more "logical volumes" using our volume group storage pool. The logical volumes are created from volume groups, and may have arbitary names. The size of the new volume may be requested in either extents (-l switch) or in KB, MB, GB or TB ( -L switch) rounding up to whole extents.
# lvcreate -l 50 -n my_logical_vol my_vol_grp
The above command allocates 50 extents of space in my_vol_grp to the newly created my_logical_vol. The -n switch specifies the name of the logical volume we are creating.
Now you can check if you got the desired results by using the command :
# lvdisplay
which shows the information of your newly created logical volume.
Once a logical volume is created, we can go ahead and put a filesystem on it, mount it, and start using the volume to store our files. For creating a filesystem, we do the following:
# mke2fs -j /dev/my_vol_grp/my_logical_vol
The -j signifies journaling support for the ext3 filesystem we are creating.
Mount the newly created file system :
# mount /dev/my_vol_grp/my_logical_vol /data
Also do not forget to append the corresponding line in the /etc/fstab file:
#File: /etc/fstab
/dev/my_vol_grp/my_logical_vol /data ext3 defaults 0 0
Resizing Logical Volumes
This is a continuation of my earlier post Creating Logical Volumes in Linux . Here I will explain how to resize an existing logical volume. Logical volumes may be resized dynamically while preserving the data on the volume. Here is how:
Reducing a logical volume
Reduce the filesystem residing on the logical volume.
Reduce the logical volume.
For different file systems, it is achieved differently.
For ext2 file system
If you are using LVM 1, then both the above steps could be acomplished by executing a single utility called e2fsadm.
# umount /data# e2fsadm -L -1G /dev/my_vol_grp/my_logical_vol# mount /dataThe above command first reduces the filesystem in the 'my_logical_vol' by 1 GB and then reduces the my_logical_vol itself by the same amount.
If you are using LVM 2 - more recent linux distributions like Fedora use LVM 2 - then you do not have the 'e2fsadm' utility. So you have to first reduce the filesystem using 'resize2fs' and then reduce the logical volume using 'lvreduce'.
# umount /data# resize2fs /dev/my_vol_grp/my_logical_vol 1G# lvreduce -L 1G /dev/my_vol_grp/my_logical_vol# mount /dataIn the above case, I have reduced my file system "to" 1 GB size ...
Note: I didn't use the minus (-) sign while using resize2fs
... And then used the lvreduce command to reduce the logical volume "to" 1 GB. If I want to reduce the logical volume "by" 1 GB, then I give the same command but with "-L -1G" instead of "-L 1G".
Reiserfs file system
If you have a reiserfs filesystem, then the commands are a bit different than ext2(3).
# umount /data# resize_reiserfs -s -1G /dev/my_vol_grp/my_logical_vol# lvreduce -L -1G /dev/my_vol_grp/my_logical_vol# mount -t reiserfs /dataXFS and JFS filesystems
As of now, there is no way to shrink these filesystems residing on logical volumes.
Grow a Logical Volume
The steps for growing a logical volume are the exact opposite of those for shrinking the logical volume.
1.Enlarge the logical volume first.
2.Then resize the filesystem to the new size of your logical volume.
Update (July 22nd 2005) : I came across this very interesting article on LVM at RedHat Magazine which I found really informative.
Posted by Ravi
No comments:
Post a Comment