Welcome to the final installment of the Advanced filesystem implementor's guide! In this article, Daniel picks up where he left off in his EVMS introduction and guides you through the process of using evmsn, EVMS' ncurses-based administration tool. He shows you how to use evmsn to take a new hard drive, partition it, and create LVM volumes on it. Along the way, he fills you in on important EVMS concepts that you'll find essential as you continue your exploration of this powerful technology.

A new disk

The best way to start playing with EVMS is to find a spare hard drive and install it in your system. Then, you can create and delete partitions and volumes to your heart's content without putting any of your data at risk. Below, I've followed this approach by booting a Gentoo Linux Game CD (see Resources later in this article), which actually happens to include EVMS 1.0.1 support. I booted the CD using vmware (check Resources for link) and configured my virtual PC to have a single SCSI disk, sda. When greeted with the login: prompt, I logged in to the live Gentoo Linux CD-based system as root, hitting Enter at the password prompt. Then I typed evmsn to start the ncurses version of the EVMS administration interface. Of course, you can also simply plug a new hard drive into your existing EVMS-enabled system if you so choose. In addition, if you have X available, then you may want to consider using evmsgui, the GTK+-based EVMS administration interface. Although evmsn runs on the console using ncurses, and evmsgui uses a more modern GTK+-based interface, the interfaces of the two programs are very similar. Experience with one should be easily transferable to the other.

Now, let's get started with EVMS. Since I'm using the console, I type evmsn and the following screen appears:

On this screen, I can see /dev/evms/sda, the only volume on my system. If you're running EVMS on your own box, you'll almost certainly see at least two additional /dev/evms/???? entries listed. You can move between each device node by using the up and down arrow keys.

You're probably wondering what these /dev/evms/??? device nodes are. At first glance, it probably appears that they are simply copies of the respective device node in /dev; for example, /dev/evms/sda simply refers to the same disk as the more traditional /dev/sda device node. So, what's the point of creating duplicate device nodes in the /dev/evms/ directory? The answer is that one of EVMS' jobs is to create a unified namespace for all the volumes on your system, and it creates this namespace under /dev/evms. So, EVMS detected sda and recognized that it should be part of the EVMS namespace, so it created the appropriate device node in /dev/evms. Now would be a good time to mention that if you aren't using devfs, you may need to run the evms_devnode_fixup program to bring your /dev/evms namespace up-to-date.

Here's another subtle yet important factoid. While EVMS can see all the storage resources on your system, it may not create /dev/evms/ device node entries for all of the resources it finds. For example, my development system has two hard drives in it, /dev/hde and /dev/hdg. However, there are no corresponding /dev/evms/hde and /dev/evms/hdg device nodes. When I look in /dev/evms/, the only device node I see is one that represents the boot partition on my first hard disk -- /dev/evms/hde1. Now, why is there a /dev/evms/hde1 but no /dev/evms/hde? Well, it turns out that EVMS is quite smart and recognizes that hde and hdg aren't set up to be volumes in themselves; instead, EVMS can see that I've partitioned hde and hdg. It can also see that I've used these partitions to create LVM logical volumes. The space on my /dev/evms/hde2 partition is being used for a higher-level storage object, so EVMS doesn't create the "/dev/evms/hde2" device node. Instead, it recognizes the LVM volumes that I've created and creates device nodes for them under /dev/evms/lvm:

		
$ ls /dev/evms/lvm/*/* -l
brw-rw-rw-    1 root     root     117,   2 Dec 31  1969 /dev/evms/lvm/mainvg/root
brw-rw-rw-    1 root     root     117,   3 Dec 31  1969 /dev/evms/lvm/raid0vg/swap
brw-rw-rw-    1 root     root     117,   4 Dec 31  1969 /dev/evms/lvm/raid0vg/tmplv

This demonstrates an important principle that EVMS follows: it will only create device nodes for the final (not intermediate) storage objects that it finds on your system. EVMS calls these final storage objects "logical volumes," whether they happen actually to be disks, partitions, or LVM logical volumes. As we'll see later, when we use the evmsn and evmsgui administration tools, we need to explicitly create an EVMS volume from a storage object before we can see a corresponding device node for it in /dev/evms/. Until we transform a storage object into a volume, EVMS will refrain from creating a device node for it, assuming that we do not intend to use it directly but instead plan to use it to create a higher-level storage object. By insisting that we create a volume before a /dev/evms device node is created, EVMS protects us from typing mke2fs /dev/evms/foo and creating a filesystem on a device node that we shouldn't be touching. Because of this behavior, we can have some assurance that every device node under /dev/evms/ holds or is intended to hold either a filesystem or swap space.

While this behavior is very sophisticated, helpful, and reduces clutter, it does take some getting used to. To drive this point home, here's a screenshot of evmsgui running on my development workstation. As you can see, only four of my storage resources have been configured as volumes, and they are all being used for filesystems or swap.

Now, back to my example vmware EVMS configuration. If you look at the first screenshot of this article, you'll notice that /dev/evms/sda is showing up as a volume even though the disk is empty. This is just another example of EVMS being cautious; while it couldn't detect any valid types of storage resources on our disk, it marked the entire disk as a volume so that we would have to delete the volume before we can start using the storage on the disk. This is a very good behavior, since the disk theoretically could contain some important data that EVMS simply can't recognize. By taking this approach, EVMS avoids the risky behavior of advertising the disk as being empty and thus possibly fooling an administrator into overwriting a raw FreeBSD volume, for example. Again, EVMS is sophisticated, but its behavior can be a bit confusing at first glance.

Of course, in my particular situation /dev/emvs/sda is empty, and I need to partition it. To do this, I need to destroy the /dev/evms/sda volume as follows. First, with /dev/evms/sda selected, I hit Enter:

As you can see, this brings up a sub-menu where I have the option of viewing details about the volume, removing the volume from the storage object, or destroying the volume. There's an important distinction between "destroy volume" and "remove volume from storage object;" if I choose the former, then my volume (as well as any sub-objects underneath it) will be broken down into their base components. However, if I choose to simply remove the volume, then the volume will be destroyed but sub-objects (if any) will remain intact. The difference between "remove" and "destroy" is analogous to peeling a layer off an onion versus throwing the entire thing into a food processor.

However, in this particular case, there is only one layer to my onion, so both options will do the same thing. I decide to choose "Destroy the volume;" after confirming my selection and hitting any key to return to the volumes listing, you now see the result of my action: "No logical volumes found."

This particular view suddenly became very boring, so I'll change it. Hitting "4" will pop up a sub-window allowing me to select a view. Right now, I'm in the "Logical volumes" view; now let's move over to "Available topmost storage objects." After making my selection, we can now see "sda" in our listing again! But unlike before, we're not looking at a volume, but a storage object. If you're following along and hit Enter on this storage object, you'll see that we could create several things from "sda", including an EVMS or compatibility volume.

However, we really don't want to do this -- if we were to create a volume from sda, we'd be back where we started when we first loaded evmsn, with a single volume that encompasses the entire disk. Instead, I want to create partitions and then convert the partitions to volumes. To do this, I hit "4" for (select view) and then select "Logical disks." Here I see "sda," the logical disk. And yes, there is an important distinction between "sda" the storage object and "sda" the logical disk. Addressing "sda" as a storage object, we can turn it into a volume or use it to build a higher-level storage object. But addressing "sda" as a disk allows me to split the disk up into partitions (called "segments" in EVMS-speak), which in turn can be used as storage objects. To do this, I hit Enter, popping up a menu that gives me the option of assigning a segment manager to the disk:

Now, if segments are the same things as partitions, you may be wondering what a segment manager is. Think of a segment manager as a "partitioning method" or scheme. Once we choose a segment manager, EVMS will put some metadata on our disk, creating a master boot record and marking the rest of the space on our disk as "freespace," available for new segments. To make this happen, I choose "DefaultSegMgr" from the menu on my screen, hit Enter again, and press space. Select "sda" as the object to receive a segment manager. Then, I hit Enter again, choose a disk type of "Linux" ("OS/2" is also available as an option), and hit Enter to continue. After pressing a key, I now hit "4" (select view) and select "Disk Segments" to see the new segments that I've created:

My disk now has a master boot record and I'm ready to create new partitions, called "segments" in EVMS-speak. As I create new segments, my "sda_freespace1" segment will shrink; if I create partitions out of all of the free space on my disk, the sda_freespace1 segment will completely disappear. It's also possible that a tiny remainder sda_freespace1 will continue to exist if the available storage on the hard drive does map cleanly to the partition sizes that I choose.

Now, to create those segments, I hit Enter when hovering over "sda_freespace1" and then select "Create a New Segment." Next, I select the DefaultSegMgr plugin, and accept the use of the sda_freespace1 object by hitting space, then Enter. Next, I'll create a small bootable partition at the beginning of the disk to hold my kernels. To do this, I'll hit space twice and then type "100" to set the partition size to 100 MB, then set "Bootable" to true by hitting space twice over the Bootable option. Finally, I'll hit Enter to create the new partition. By repeating these steps, I then create other partitions for my filesystems and swap. Some notes: if you need to create more than four partitions, remember to toggle the "Primary Partition" option after creating your boot volume. Also remember to set the "Partition Type" to "Linux Swap" when creating your swap partition.

The final step is to hit Enter over "sda1," "sda2," and "sda3." A menu appears showing an option that reads "Create Compatibility Volume from the Segment." When I select this option, my segment will be configured to house a full-fledged logical volume, meaning that it will now have a device node under /dev/evms/. In addition, because I chose to create a compatibility volume, the new volume will be usable even if I happen to reboot my system with a non-EVMS kernel. If I had chosen "Create EVMS Volume," then this wouldn't have been the case. Note that EVMS versions 1.1.0 and later allow you to convert compatibility volumes into EVMS volumes, and vice-versa.

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Conclusion

So there you have it, I've partitioned a single system disk using EVMS. While not an earth-shattering accomplishment, such an experience will help to give you the EVMS foundation you need before attempting things like creating storage containers, LVM logical volumes, RAID volumes, snapshots, and other goodies. Of course, I didn't have room to cover these topics in this article, but I hope to be able to touch on some of these EVMS applications intermittently throughout the next year. But first, on to other things; it's time to bring the Advanced filesystem implementor's guide to a close. I hope you've enjoyed the ride :)

Resources

• Daniel introduced EVMS in Part 12 of the Advanced filesystem implementor's guide, "Introduction to EVMS" (developerWorks, Octorber 2002).
  
  
  
  
• Visit the official development home of the EVMS project on SourceForge, where you can subscribe to mailing lists and read EVMS documentation, among other things.
  
  
  
  
• Visit the #evms channel on the freenode (formerly Open Projects Net) IRC network. Several EVMS developers regularly hang out in #evms.
  
  
  
  
• VMware is a virtual machine software package.
  
  
  
  
• Learn more about Gentoo Linux.
  
  
  
  
• An easy way to begin exploring EVMS is to download a Gentoo Linux Game/Install mini-CD -- grab livecd-ut2003-i586-1.4.1.iso from the Gentoo 1.4 x86 download directory.
  
  
  
  
• Find related information on Linux Software RAID in Daniel's series:
  • Part 1: Introduction and installation (developerWorks, February 2001)
  • Part 2: Setup in a production environment (developerWorks, February 2001)
  
  
  
  
• Daniel explores Linux Logical Volume Management in:
  • Part 1: Learning LVM (developerWorks, March 2001)
  • Part 2: The cvs.gentoo.org upgrade (developerWorks, April 2001)
  
  
  
  
• Find more Linux articles in the developerWorks Linux zone.
  
  

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Table of contents

• A new disk
• Conclusion
• Resources
• About the author
• Comments

Local resources

• IBM Innovation Center - Austin, TX
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