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As I said in an earlier Tales from the Field, when you write a book like this one, people assume you are an expert on every issue related to the topic, and so you get pulled into all sorts of technical debates and arguments. Some are interesting, some are hairsplitting. One of the more interesting (and yet hairsplitting) debates was over which name, 0+1 or 1+0, actually referred to mirrored stripes, and which referred to striped mirrors, and for that matter, whether mirrored stripes meant that the mirroring took place first or the striping did. The nomenclature that we use in this section was based on some research, as well as on what we see in marketing literature from companies who have products that actually deliver RAID 1+0 and 0+1. My worthy opponent in the debate was a colleague, friend, and prolific author, Paul Massiglia, who served on the original RAID Advisory Board, and who had helped name the levels of RAID, including 1+0 and 0+1, back in 1989. The Board had, unfortunately, named them the other way around, and Paul and I were being asked to reach a consensus on the names for VERITAS s marketing department. We agreed that the names striped mirrors and mirrored stripes were very confusing, largely because it s hard to remember which is which (and the numbers don t help either). When the board assigned the names, they decided that 0 (striped) + 1 (mirrors) was a sensible naming convention. Unfortunately, the convention evolved differently in the marketplace. In the end, the argument came down to style rather than any technical issue. We decided to use the style that you see in this book, after a discussion that ran on and off for several months and that was briefly revived (for about the 10th time in the last few years!) as I put this Tale together. Evan
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The difference in layouts between the two models depends on which of the two methods is employed first. As shown in Figure 7.6, in RAID 0+1 (or mirrored stripes), the stripes on one side of the mirror are built first and then they are mirrored to the other side. Figure 7.7 shows RAID 1+0 (or striped mirrors); the mirrors are built separately first, and then are striped together as if they were separate disks.
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1. Select disks 2. Stripe the disks together 3. Select more disks 2GB 2GB 4. Stripe them together 5. Mirror the sets together 2GB
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2GB 6GB Stripe Set 2GB 6GB RAID 0+1
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Figure 7.6 Building RAID 0+1: Mirrored stripes.
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The differences between 0+1 and 1+0 are not particularly apparent in the description of how they are built. But when you look at how they handle failures and how they recover from those failures, the differences, as well as the advantages of one over the other, become much more striking.
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1. Select Disks 2GB 2GB 2. Mirror them in pairs 3. Stripe the Mirrors together 2GB 2GB 6GB RAID 1+0
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Figure 7.7 Building RAID 1+0: Striped mirrors.
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As shown in the RAID 0+1 example (see Figure 7.8), the loss of any member of a stripe causes the loss of the entire stripe. If disk A, B, or C fails, all of stripe 1 is lost. If disks D, E, or F fail before the first failed disk can be recovered and the data refreshed, the entire data set is lost. In the RAID 1+0 example in Figure 7.9, if A or B fails, there is no data loss. If C or D fails, there is no data loss. If E or F fails, there is no data loss. However, if A and B fail at the same time (or C and D, or E and F), then and only then the entire RAID device is lost. One might note that either RAID composition could sustain the loss of one half of its disk members and still survive. However, the odds for surviving are very much in favor of 1+0. If disk A fails in either figure, both compositions will survive. In the RAID 1+0 example, of the five disks that remain after disk A fails, the failure of only one of them (B) could cause a catastrophic outage. In the RAID 0+1 example, if A fails, then a catastrophic failure will result if D, E, or F fail. So, in our examples, the chance that a second disk failure will cause a catastrophe in RAID 1+0 is only 20 percent (1 in 5), while in 0+1 the chance is 60 percent (3 in 5). And since B and C are not actually in use once A fails, the chances for catastrophe are even greater. The other issue when comparing the two combined RAID levels is recovery time. Once disk A has failed in RAID 0+1 (Figure 7.8), and the entire stripe is lost, all three disks in the stripe (A, B, and C) must be synced with D, E, and F in order to bring stripe 1 back on line. To recover from the loss of disk A in the RAID 1+0 example (Figure 7.9), only disk A needs to be resynced. The reliability advantage of 1+0 over 0+1 is clear. The only real downside to 1+0 is that each of the stripe components needs to be the same size. That can be inconvenient when designing the layout. RAID 0+1 does not have that requirement, and only requires that all sides of the mirror be the same size.
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Stripe 2 1. Disk A fails A D 2. All of stripe 1 is lost; data remains available via stripe 2 3. Disk E fails 4. Stripe 2 is lost; data is no longer available C F
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