Today we're going to set up volume groups using the disks, and disk groups, that we setup yesterday in our post on creating disk groups with Veritas Volume Manager.
Our assumptions for the disks, and disk groups, we've setup are also spelled out in that same post about initializing disks and creating those disk groups. If you have any questions regarding the setup we're working from, they should be answered there :)
Now, we're ready to take the disk groups we created yesterday (again, not recommended to have all 3 of those exist at one time in reality. But highly recommended for the purposes of explaining the basic commands and concepts in today's post ;)
We're going to create the 4 most basic types of volumes today. These are referred to by their RAID (Redundant Array of Indepedent Disks) numbers, which translate as follows:
RAID 0 = Striped/Concatenated disk <--- Use this if you just want to combine any number of disks into one giant volume. Stripes of data are written across multiple disks, but this only enhances usability and doesn't provide any protection from disaster if a disk in the volume group fails.
RAID 1 = Mirrored disk <--- Use this if you want to mirror your disk, so that you have a simple mirror to fail over to if your primary disk fails.
RAID 5 = Striped disk with parity <--- Use this if you want to make your volume group highly available and recoverable. You'll need at least 4 disks in your RAID 5 group for this setup to be worthwhile, but it can make uptime and recovery a lot less of a hassle if you've got the disk to throw at your storage needs.
RAID 1+0 = Concatenated Mirrored disk <--- (as opposed to Mirrored Concatenated disk). The distinction to be made here is that RAID 1+0 (Striped Mirrors) consists of several disk mirrors, each reflection of which is concatenated/striped to create a larger volume. In RAID 0+1 (Mirrored Stripes), you have a set of disks Concatenated/Striped first and then Mirrored back to an equal amount of concatenated disk. Hopefully, I'm explaining this well enough, and not over-explaining it ;) Basically, with RAID 1+0, you can afford to lose a lot more disk and remain "up" than you can with RAID 0+1 (In RAID 0+1, loss of any disk A on one side of the mirror and disk C on the other side will bring you down, since each loss of a single disk, on either side, takes down that whole Concatenated miror. RAID 0+1 is like creating a much larger simple Mirrored disk than you could create with RAID 1).
For the following examples, we're going to use the command "vxassist." While you can use vxmake, etc, to perform these same functions (and, eventually, may have to if you need to do things more precisely), using vxassist is optimal when you're working with fresh disk and want to get up and running quickly :)
1. Creating a RAID 0 Volume Group named volume_group1 (From disk_group2, which contains hdc, hdd, hde and hdf):
host # /usr/sbin/vxassist -g disk_group2 -b make volume_group1 768000s layout=striped stripeunit=128 ncolumn=4
2. Creating a RAID 1 Volume Group name volume_group2 (from disk_group1, which contains hda and hdb):
host # /usr/sbin/vxassist -g disk_group1 -b make volume_group2 512000s layout=nostripe,log nmirror=2
3. Creating a RAID 5 Volume Group name volume_group3 (from disk_group3, which contains all 6 disks):
host # /usr/sbin/vxassist -g disk_group3 -b make volume_group3 1280000 layout=raid5,log stripeunit=32 ncolumn=6
4. Creating a RAID 1+0 Volume Group name volume_group4 (from disk_group3, which contains all 6 disks):
host # /usr/sbin/vxassist -g disk_group3 -b make volume_group4 768000s layout=stripe-mirror,log nmirror=2 stripeunit=128 ncolumn=2
As a bit of explanation, for each instance of vxassist that we invoke, you'll notice some common elements:
The "-g" flag simply asks us to name the disk group that we'll be using to create the volume group.
The volume group name (after "-b make") is simply what we want to call the particular volume group we're creating.
The following field is the size of the volume in sectors.
The field after that is the "layout" that we're looking to create. For RAID 0, we chose "striped." For RAID 1, "nostripe, log." For RAID 5, "raid5, log." And, for RAID 1+0, "stripe-mirror,log."
The "stripeunit" is only required for striped volumes and you can leave this out if you want to accept your VVM version's default.
The final column is also variable. For RAID 0, 5 and 1+0 we have the "ncolumn" attribute which basically translates to the number of disks in our volume group. For RAID 1 and 1+0 we have the "nmirror" attribute which translates to how many mirror devices we want to have. Two mirrors is actually the minimum you can have and really means "one disk, mirrored to one other disk." I initially found that confusing. Assuming I'm not the only guy who wondered if I was triple-mirroring by specifying two mirrors, I thought I'd just add that on there ;)
Hopefully this has been helpful to you. I'll be writing more on this, Solaris Volume Manager and LVM for Linux in posts to come :)
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Saturday, March 1, 2008