VERITAS is a logical volume manager. The operative word here is 'logical'. So, in order to keep things in order we have some basic pieces which are used in different ways for building volumes. The better you understand these guys, the better you'll understand VERITAS. Here's the list of objects:
Disk: This is a normal physical disk with a SCSI id. (c0t0d0...)
VM Disk (dm): A disk that is put under Vx control.
Sub Disk (sd): A section of VM disk used to build plexes.
Plex (pl): A mirror.
Volume (v): A virtual disk, which can contain data.
Now, lets talk about these objects a bit. A disk is nothing new or magical. When we want to use a disk in VERITAS we need to turn it over to VERITAS control. A disk turned over to VERITAS control is then given a VERITAS name (like disk01). After doing this the disk is no longer available to the system for anything outside of use inside VERITAS. Also, when we turn disks over to VERITAS control we don't turn over a partition (c0t0d0s0), but the whole disk itself (c0t0d0). Now that we've got a VM Disk we then create a Sub Disk from the VM Disk. Think of a subdisk as a VERITAS partition. We could make one big subdisk from the VM Disk and use the whole disk, or we could create a bunch of smaller subdisks. You can divide VM Disks into subdisks however you like. From subdisks (one or more) we create what is called a plex. Plexes are confusing so let's talk about these in some length.
Veritas™ Volume Manager Administrator's Guide Thesoftwaredescribedinthisbookisfurnishedunderalicenseagreementandmaybeused only in accordance with the terms of the.
- Tags: grow root filesystem solarisadmin veritas troubleshooting VxVM Learning Ramdev I have started unixadminschool.com ( aka gurkulindia.com) in 2009 as my own personal reference blog, and later sometime i have realized that my leanings might be helpful for other unixadmins if I manage my knowledge-base in more user friendly format.
- Displaying available paths using VxVM on Veritas You can use the VxVM management interface to display information about the VxVM devices and see which devices are managed by the VxVM The vxdisk command displays information about device, type, disk, group, and status.
- Creating Veritas Volume Manager (VxVM) Volume & Filesystem in RHEL 7. First of all, check the disks which are visible to Veritas Volume Manager (VxVM) and this can be done by “vxdisk” command as below.
Say the following out loud: 'A Plex is a Mirror. A Plex is a Mirror. A Plex is a Mirror.' Maybe you should do that a couple times. You may feel silly, but plexes are a sort of cornerstone in VERITAS. A Volume is a container, in VERITAS. The volume is made up of one or more plexes. See the catch? A Plex is a mirror, yes, however you can make a volume with only one plex. Is a volume made with only one plex mirrored? No. We'll explain this more later, but for the time being keep it in your head. So, subdisks are grouped together into a plex. The interesting thing is that in VERITAS the plexes do all the work, so lets say you wanted to create a Striped volume. You would actually create a striped plex, and then attach that striped plex to a volume. The volume doesn't care, it's just a container. See the beauty here? Let's put all this stuff together and build an imaginary volume in VERITAS.
We're going to build a striped (RAID0) volume from 2 9G disks. We'll say that the first disk is c1t0d0, and the second is c1t1d0. First, we need to put them in VERITAS control, so we create VM disks. The VM disks are then named disk01, and disk02. Next, we'll create subdisks using these two disks. Let's use the whole disks and just create 2 subdisks, one for each VM disk. We'll call disk01's subdisk 'disk01-01', and disk02's subdisk 'disk02-01'. Now, it's plex time! We're going to build a striped plex using our two subdisks, which we'll call 'myplex'. (Don't worry yet about how we create the plex, just get the concepts now.) So now we've got a plex, which contains the subdisks 'disk01-01' and 'disk02-01'. Now, we create a volume named 'myvol' using 'myplex'. And bingo! We've got a striped 18G volume ready to create a file system on! Maybe, if we used the short names mentioned earlier (with the list of objects) and make an outline it'd look something like this:
Look OK? Because if it does, take a look at this, real output from VERITAS, from a real volume I created on my test machine:
Does that make any sense? Any at all? I hope it does. And if it does, you're ready for VERITAS. We're going to explain more, much more, as we roll along, but at least understand that:
Good. One more note about plexes before we move on. Here's the groovy thing about plexes. Because plexes are 'mirrors', we can mirror the volume we built earlier simply by building another plex identical to the first one, using two more subdisks (which means we need 2 more vmdisks, etc,etc). Once we build the second plex, we attach it to the volume (myvol) and presto chango! We're mirrored! This is really kool... mirrors are something magical, they are a part of everything. If you have only one mirror you may see yourself, but you'll need a second mirror to see yourself holding the mirror. Hmmm?
VxVM Tutorials
Let us now see different volume layouts that can be created in veritas volume manager. Before diving deep into the RAID levels first create a Disk group with 4 disks. To do this we have to initialize 4 disks to take them under VxVM control.
After this create the disk group mydg with these four disks by naming them from disk01 through disk04.
To confirm the disk group creation check the disks in the disk group mydg:
Concatenated Volume
As shown in the diagram below, in RAID 0 concat volume data is first written linearly in disk01 and then to disk02. Number of subdisks in a concat volume depends upon the size of the volume. IF you have a volume greater than the size of a physical disk you would have a single sub disk on one physical disk and another sub disk on the next physical disk. Remember that the number of sub disks also depend on other factors too. The disadvantage of concat volume is that, it does not provide any redundancy to the data as we always have only one copy of data at a time. Minimum disks required to create a concat volume is 1. Command to create a concat volume :
To print the layout of myvol :
RAID 0 (striped volume)
In this case we spread the data out to multiple disk by creating equal sized columns on each disk. Each column will be on a separate disk. Similar to concat volume a stripped volume also do not provide any redundancy to the data as it has only one copy of data at any point in time. To create a stripped volume you need atleast2 disks.
Now you can specify how many columns you need to stripe out the data. Also you can mention the stripe unit.
Mirrored layout (RAID 1)
Mirrored layout or RAID 1 layout is one of the most commonly used layout in production environments. RAID 1 simply mirrors all the data from one disk to another so that we have an extra copy of data. This provides us the necessary data redundancy. Here each plex is on a different disk. Command to create a RAID 1 mirrored volume is :
To check the layout of a RAID 1 mirrored volume :
RAID 5
In case of RAID 5 volumes a parity information is stored on every disk as shown in the diagram. Now this parity can be used in case you want to recover the data lost due to a disk failure. The parity is spread across all the disk to ensure that we have sufficient copies of parity all the time. We need atleast 3 disks to create a RAID 5 volume. Command to create a RAID 5 volume is :
To check the layout of a RAID 5 volume :
Advantages and disadvantages of various RAID levels
| Concat | RAID 0 (striping) | RAID 1 (Mirror) | RAID 5 | |
|---|---|---|---|---|
| Advantages | 1. Easy to administer 2. Maximum utilization of disk space 3. Any size disks can be added to create volumes. | 1. Parallel data transfer to improve write speed. | 1. Protection against disk failure 2. Faster write performance | 1. More usable disk space than mirroring 2. Redundant 3. Fast read 4. Faster recovery through parity. |
| Disadvantages | 1. No redundancy | 1. No redundancy | 1. Requires more disks to configure 2. A bit slow write performance. | 1. Slow write due to parity. 2. Poor performance after disk failure. |
Mirror-stripe (RAID 1+0)
In case of a stripe mirror the data is first mirrored and then the mirrored data is striped as shown in the diagram. This requires atleast 4 disk. Command to create a mirror-stripe is :
To print the layout of a mirror-stripe volume :
Mirror-concat (RAID 0+1)
Now in case of a mirror-concat volume, the data is first concatenated and then this concatenated data is mirrored as shown in the diagram. This requires atleast 4 disks. To create a mirror-concat volume :
To print the layout of a mirror-concat volume :
Comparisons of various RAID levels
| concat | stripe (RAID 0) | mirror (RAID 1) | RAID 5 | RAID 1+0 and RAID 0+1 | |
|---|---|---|---|---|---|
| Minimum number of Disks required | 1 | 2 | 2 | 3 | 4 |
| Data Protection | no | no | yes | yes | yes |
| Disk space usage (Using minimum number of disks) | 100% | 100% | 50% | 67% | 50% |
| Read performance | High | High | High | High | High |
| Write performance | High | High | Medium | Low | Medium |
Layered volumes
Layered volumes are actually built on top of other volumes. In layered volumes data is mirrored at sub disk level which brings in more granularity. Now to do this VxVM creates sub volumes from traditional volumes. Advantages 1. Faster recovery from failure than RAID 1+0 and RAID 0+1 2. Can persist more disk failures than RAID 1+0 and RAID 0+1 Disadvantage 1. Requires more VxVM objects ( sub volumes are required for a layered volume)
1. concat-mirror
In case of concat-mirror, sub disks are concatenated to create the plexes for the sub volumes. The sub volumes are the mirrored to create the top level plex and eventually top level volume. Advantage Recovery is faster as each portion of concatenation is mirrored separately Command to create a concat-mirror volume :
Check the layout of the volume we just created. You should see 2 subvolumes with 2 volumes each. Each volume will have one plex with one sub-disk.
Veritas Vxvm Download
2. stripe-mirror
In case of stripe-mirror, sub disks are mirrored to create the plexes for the sub volumes. The sub volumes are then striped to create the top level plex and eventually a top level volume. Advantage More tolerant to faults, as each stripe is mirrored separately. Command to create a stripe-mirror volume :
Check the layout of the volume we just created. You should see 2 subvolumes with 2 volumes each. Each volume will have one plex with one sub-disk.
Veritas Vxvm Cheat Sheet
I hope the post was informative, Do subscribe to stay updated.