A greatly powerful, and often requested, function of Veritas Volume Manager if often the least well understood. This being the usage of Layered Volumes. Working with RAID0+1 (Stripping plus Mirroring), RAID0 (Stripping), and RAID5 are well understood concepts for most. But when it comes to the popular alternative of RAID1+0 (Mirroring plus Stripping) we find a knowledge rift due to poor documentation on the part of Veritas. Well, in this document we're going to fix that. But first, what is RAID1+0?
In a "typical" RAID0+1 volume configuration we take several disks and then create a stripe across thoughs disks (the RAID0 part). Then once complete we do this again on a separate set of disks, and then attach that new stripe to the first creating a mirror (the +1 part). We then have a RAID0+1 volume thats ready to have a filesystem put on it. The point of interest with this setup is that we're actually mirroring a complete stripe (and therefore ALL the disks in that stripe) to another stripe (and therefore ALL of it's disks). The problem here is that if for some reason we need to re-sync the volume we'd need to re-sync a full stripe to a full stripe (very timely) which is a nearly tragic proposition if your talking about 50G+. A far more efficient setup would be to mirror each disk to each disk... in other words, to mirror a bunch of disks on a one-to-one basis, and then build a stripe on top of these mirrors. In this case if we need to re-sync due to a disk failure we can simply sync the failed disk to its mirror, instead of the full stripe. This is the power of RAID1+0; the difference between mirroring the stripes (0+1) and stripping the mirrors (1+0).
If the terms seem to confuse you, try this for size:
RAID0 Striping (VxVM says: stripe) RAID1 Mirroring (VxVM says: mirror) RAID0+1 Striping plus Mirroring (VxVM says: mirror-stripe) Think this: Striped disks, then mirror the stripes RAID1+0 Mirroring plus Striping (VxVM says: stripe-mirror) (Veritas Marketing Dept says: StripePro Think this: Mirrored disks, then stripe on top of the mirrors Concat+Mirror Concatenation plus Mirroring (VxVM says: mirror) Same as RAID1 Mirror+Concat Mirroring plus Concatenation (VxVM says: concat-mirror) (Veritas Marketing Dept says: ConcatPro) Think this: Concatenation on top of mirrored disks. |
The difference between the different types of combinations of concatenation, striping and mirroring are really just a matter of what is done first. Do you mirror subdisk and then stripe on top of thoughs mirrors or do you stripe several subdisks and then mirror the full stripe? Sometimes thinking "Stripe Then Mirror" is more clear than "Striping And Mirroring". The terms can be confusing, and I guaranty you'll fight with someone about it eventually, but luckily with the nifty terms StripePro and ConcatPro, not only is there no argument of 1s and 0s, but it sounds techy too! When working with layered volumes just remember that, at its heart, its just the ability to embed volumes inside volumes.
Clear? Lets look the actual process of building these RAID1+0 volumes.