Learning Linux LVM, Part 2In my first LVM article, I explained the concepts behind LVM. Now it's time to put LVM into action. In this article, I'm going to set up LVM on the official Gentoo Linux cvs server, cvs.gentoo.org. Although cvs.gentoo.org has only one hard drive, LVM's flexibility still provides an incredible improvement over the standard static partitioning approach. I'll show you all the steps of the LVM conversion process, so that if you're interested you can perform a similar conversion on one of your machines.
That said, let's continue. Before starting the conversion process, I upgraded cvs.gentoo.org so that it was using the following packages. At the time I performed the LVM transition, these were the latest versions available (see Resources later in this article):
Now, for the hard drive. cvs.gentoo.org had a nice new IBM 45 GB hard drive sitting in it; however, when I installed Gentoo Linux on cvs, I only partitioned about 10 gigabytes of the drive, keeping the remaining 35 GB for future partitions. Such are the little tricks you need to employ when not using LVM -- leaving part of the drive unpartitioned is a primitive but effective way to allow for future expansion. However, with LVM there is a better approach. In the past few weeks, I had been noticing that my root ReiserFS partition had been slowly filling up, as you can see from this df output:
Now, a 72% full root partition isn't exactly a crisis, but it isn't a wonderful situation either. ReiserFS, like many other filesystems, starts slowing down as it gets more and more full, and it was just a matter of time before my root filesystem would fill up completely and filesystem performance would take a hit. I decided to fix this problem by using LVM to create a new logical volume out of the 35 GB of currently unpartitioned space at the end of my hard drive. Then, I'd create a filesystem on this volume and move a good chunk of the contents of /dev/hda3 to it. If you're thinking of making a similar transition on one of your machines, the first thing you need to do is find a suitable piece of your root filesystem to move to a logical volume. For me, the choice was easy -- my /home tree was taking up around 5.7 GB. By moving /home to its own LVM logical volume, my root filesystem would then be at about 20% capacity. Since most new data is being added to /home, my root filesystem would likely stay at around 20% capacity as well -- a very healthy situation. To begin the conversion, I first had to partition the unused space at the end of my hard drive. Using cfdisk, I created a 35 GB partition (/dev/hda5) and set the partition type of the partition to 8E (the official LVM partition type). After this change, I rebooted to force a reread of my partition table. After the reboot, my partition table looked like this:
Now that I had an empty 35 GB partition, I was ready to initialize it for LVM. Here's the procedure -- first, I would initialize the 35 gigabytes as a physical volume; then, I would create a volume group using this physical volume, and finally, I would allocate some of the extents on the volume group, creating a logical volume that would contain my new filesystem and house all the files currently in /home. To begin the process, I used the pvcreate command to initialize /dev/hda5 as a physical volume:
pvcreate set up a special "accounting" area on /dev/hda5, called the VGDA (volume group descriptor area). LVM uses this area to keep track of how the physical extents are allocated, among other things. My next step was to create a volume group and add /dev/hda5 to this group. The volume group would act as a pool of extents (chunks of storage blocks). Once the volume group was created, I could create as many logical volumes as I wanted. I decided that my volume group would be called "main":
The vgcreate command did a couple of things. In addition to creating the "main" volume group, it also set up /dev/hda5 to use 4 MB extents, the default extent size. This means that any logical volumes I create from this volume group can be expanded and shrunk in 4 MB increments. Due to kernel limitations, the extent size determines the maximum size that a logical volume can be. As you can see from the above output, a 4 MB extent size imposes a logical volume size limitation of 256 gigabytes, which is an easily attainable logical volume size if you're adding several high-capacity drives to your volume group. If your volumes could end up being greater than 256 GB apiece, I recommend specifying a larger extent size at vgcreate time. Extents can range anywhere from 8 KB to 512 MB, and must always be a multiple of two. By increasing the extent size above 4 MB, the maximum physical volume size will be scaled accordingly, up to a maximum of 1 petabyte (although the current real-world size limit is 2 terabytes on x86 systems). For example, if I wanted to create a volume group with 32 megabyte extents, I'd type:
32 MB is a good extent size, since a 32 MB granularity is still manageable and pushes the maximum logical volume size to 2 terabytes to boot. Once your volume group is created, you can view its information by typing vgdisplay:
Now that I had my volume group, I was ready to create a logical volume. I decided to initially make it 8 gigabytes in size and call it "lv_home":
Then, I created a filesystem on the volume:
Now that the filesystem was created, I could mount it at /mnt/newhome:
As you can see above, I was almost ready to copy over all my data in /home. Before I began, I dropped to runlevel 1 to ensure that no users or processes would be accessing or modifying files in /home as they were being copied over:
Then, I began copying the files:
The copy completed in about ten minutes. Then, I backed up my original /home to /home.old, just in case something was wrong with my copy. I created a new mount point, and remounted the new home at /home:
Then, it was time to set up the server so that my new /home partition would be available every time the machine started up. First, I modified my /etc/fstab so that it included a new /home entry:
Then, I made minor modifications to my initialization scripts. I modified my "checkroot" startup script so that the following commands would run immediately after my root partition was remounted read/write:
Then, I modified my filesystem unmounting script that gets run on shutdown, so that the following command would run immediately after all filesystems were unmounted:
Once I had completed these steps, I rebooted the machine, and to my delight everything worked perfectly. After a day or so of absolutely no problems, I deleted /home.old to free up some space on my root filesystem. Yay! The transition to LVM was a success. While the transition to LVM is a bit of an ordeal, once the transition is complete, managing filesystems becomes tremendously easier. As an example, I decided to resize my new /home logical volume, adding about 2 gigabytes worth of space to the end of the filesystem. First, I added additional capacity to my "lv_home" logical volume, and then I used the resize_reiserfs utility to expand the filesystem so that it would use this additional capacity. Here are the two commands that did all this:
In about a second, I had enlarged my /home filesystem by 2 GB; amazingly, I didn't need to reboot, drop to runlevel 1, or even unmount /home to perform the resize. Everything continued to work as it had before. Isn't that great? Here's the current state of my filesystems:
You can see how LVM really can make an administrator's work a whole lot easier. In the future, I hope to move additional parts of my root filesystem over to LVM, and eventually even convert my root filesystem over to an LVM logical volume. The resources below will help you learn even more about LVM.
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