VxVM Tutorials
You must have thought that, when we can create a simple RAID 1+0 or RAID 0+1 volume, why we go for layered volumes with complicated structure and sub volumes?! The answer is simple. The main reason behind using layered volumes is that, the extra redundancy they provide than the traditional RAID 1+0 or RAID 0+1 mirrors. Lets us see how are layered volumes are better than the non layered volumes.
To compare layered and non-layered volumes, we would take a mirror-stripe and stripe-mirror volume with 4 disks each and test volume state when any 2 disks fail in the volume.
Now for a volume to be available with all the data in it accessible, you need at least one plex available at a time. Consider a case when disk01 and disk04 fails at the same time. In traditional RAID 1+0 volume the volume will fail as sd1 and sd4 will not be available resulting in failure of both the plexes.
In case of layered volumes though, sd1, sd2 and sd3, sd4 are mirrored which makes failure of sd1 and sd4 ineffective. The reason being you have mirrred the sub disks in each sub volume and in this case you only loose one copy of mirror.
Now similarly you can consider all other cases of disk failures. We can easily conclude that in case of a layered volume the rate of volume failure is 4/6 or 2/3 (4 failure of volumes out of 6 cases), whereas in case of a non-layered RAID 1+0 volume it is 2/6 or 1/3. This is the reason non layered volumes were introduced in VxVM 3.0 version. But remember that the layered volume though more tolerant to failure uses more VxVM objects like sub volumes and is a bit complicated to administer.
Mirror-stripe (non-layered volume)
As shown in the figure below, we have a mirror-stripe volume with 4 disks.
| State of volume | disk01 | disk02 | disk3 | disk4 |
|---|---|---|---|---|
| Down | failed | failed | ||
| Up | failed | failed | ||
| Down | failed | failed | ||
| Down | failed | failed | ||
| Up | failed | failed | ||
| Down | failed | failed |
Stripe-mirror (layered volume)
As shown in the diagram we have a 4 disk layered volume, stripe-mirror.
| State of volume | disk01 | disk02 | disk3 | disk4 |
|---|---|---|---|---|
| Down | failed | failed | ||
| Up | failed | failed | ||
| Up | failed | failed | ||
| Up | failed | failed | ||
| Up | failed | failed | ||
| Down | failed | failed |
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