>>> I got a SMART error email yesterday from my home server with a 4 >>> x 1Tb RAID6. [ ... ] >>> That's an (euphemism alert) imaginative setup. Why not a 4 >>> drive RAID10? In general there are vanishingly few cases in >>> which RAID6 makes sense, and in the 4 drive case a RAID10 >>> makes even more sense than usual. Especially with the really >>> cool setup options that MD RAID10 offers. > In this case, the raid6 can suffer the loss of any two drives > and continue operating. Raid10 cannot, unless you give up > more space for triple redundancy. When I see arguments like this I am sometimes (euphemism alert) enthused by their (euphemism alert) profundity. A defense of a 4-drive RAID6 is a particularly compelling example, and this type of (euphemism alert) astute observation even more so. In my shallowness I had thought that one goal of redundant RAID setups like RAID10 and RAID6 is to take advantage of redundancy to deliver greater realiability, a statistical property, related also to expected probability (and correlation and cost) of failure modes, not just to geometry. But even as to geometrical arguments, there is: * While RAID6 can «suffer the loss of any two drives and continue operating», RAID10 can "suffer the loss of any number of non paired drives and continue operating", which is not directly comparable, but is not necessarily a weaker property overall (it is weaker only in the paired case and much stronger on the non paired case). This ''geometric'' property is of great advantage in engineering terms because it allows putting drives in two mostly uncorrelated sets, and lack of correlation is a very important property in statistical redundancy work. In practice for example this allows putting two shelves of drives in different racks, on different power supplies, on different host adapters, or even (with DRBD for example) on different computers on different networks. But this is not the whole story, because let's look at further probabilistic aspects: * the failure of any two paired drives is a lot less likely than that of any two non-paired drives; * the failure of any two paired drives is even less likely than the failure of any single drive, which is by far the single biggest problem likely to happen (unless there are huge environmental common modes, outside "geometric" arguments); * The failure of any two paired drives at the same time (outside rebuild) is probably less likely than the failure of any other RAID setup component, like the host bus adapter, or power supplies. * As mentioned above, the biggest problem with redundancy is correlation, that is common mode of failures, for example via environmental factors, and RAID10 affords simpletons like me the luxury of setting up two mostly uncorrelated sets, while RAID6 (like all parity RAID) effectively tangles all drives together. As to the latter, in my naive thoughts before being exposed to the (euphemism alert) superior wisdom of the fact that «raid6 can suffer the loss of any two drives and continue operating» I worried about what happens *after* a failure, in particular to common modes: * In the common case of a loss of a single drive, the only drive impacted in RAID10 is the paired one, and it involves a pretty simple linear mirroring, with very little extra activity. This means that both as to performance and to environmental factors like extra vibration, heat and power draw the impact is minimal. Similarly for the loss of any N non paired drives. In all cases the duration of the vulnerable rebuild period is limited to drive duplication. * For RAID6 the loss of one or two drives involves a massive whole-array activity surge, with a lot of read-write cycles on each drive (all drives must be both read and written), which both interferes hugely with array performance, and may impact heavily vibration, heat and power draw levels, as usually the drives are contiguous (and the sort of people who like RAID6 tend to make them of identical units pulled from the same carton...). Because of the massive extra activity, the vulnerable rebuild period is greatly lengthened, and in this period all drives are subject to new and largely identical stresses, which may well greatly raise the probability of further failures, including the failures of more than 1 extra drive. There are other little problems with parity RAID rebuilds, as described in the BAARF.com site for example. But the above points seemed to me a pretty huge deal to me, before I read the (euphemism alert) imaginative geometric point that «raid6 can suffer the loss of any two drives and continue operating» as that be all that matters. > Basic trade-off: speed vs. safety. I was previously unable to imagine why one would want to trade off much lower speed to achieve lower safety as well... :-) -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
