Mark Knecht posted on Fri, 22 Nov 2013 08:50:32 -0800 as excerpted: > On Fri, Nov 22, 2013 at 12:13 AM, Stan Hoeppner <s...@hardwarefreak.com> > wrote: >> Now that you mention it, yes, RAID 15 would fit much better with >> convention. Not sure why I thought 51. So it's RAID 15 from here. > <SNIP> > > For us casual readers & RAID users could you clarify RAID15? Would that > be a bunch of RAID1's grouped together in what appears to be a RAID5 to > the system?
Simplest definition, yes. Admittedly part of this discussion is beyond me (as another casual reader with some raid experience, reading here via the btrfs list as that's my current interest), but I'm following enough of it to find it interesting, for SURE! =:^) And perhaps my explanation of the basics will let the real experts continue the debate at their higher level... At a concept level, because md/raid, etc (I'll use mdraid as my example from here, but but there's dm-raid, hardware raid, etc; additionally, I'll omit the ALL CAPS RAID convention and use lowercase), devices are presented as normal block devices, RAID levels (among other things, LVM2, etc) are stackable. So it's possible to, for instance, create a raid0 on top of a bunch of raid1s, or the reverse, a raid1 on top of a bunch of raid0s, either with the base level being hardware based and the software creating a raid level direct on the hardware raid, or with both/all levels in software. Then we get into naming. AFAIK the earliest convention was using the plus syntax, raid1+0, raid0+1, with the left-most number being the lowest, closest to hardware level, either the hardware level or closest to the individual hardware devices, so raid1+0 is implemented as striped raid (raid0) over top of mirrored raid (raid1), with raid0+1 the reverse, a mirror over stripes. That quickly evolved into omitting the +, thus raid10 and raid01. (Tho 01 has the leading zero problem with some people trying to omit it, and raid1 isn't the same thing AT ALL as raid01! Between that and the fact that raid01 is less common than raid10 for technical reasons as noted below, you seldom see raid01 specified; it usually keeps the + and appears as raid0+1). Also, less commonly seen but as more levels were stacked (raid105, etc), sometimes the + is still used to separate the hardware raid levels from software. In this usage, raid105 would probably be an all software implementation, while raid1+05 would be raid1 in hardware, with software raid0 and raid5 stacked on top, in that order, and raid10+5 would be hardware raid10, with software raid5 on top. Note that while raid10, aka raid1+0, should have similar non-degraded performance to raid0+1, there's a BIG difference when recovering from degraded. A smart raid10 implementation (or a raid1+0 with hardware raid1) can rebuild a failed drive "locally", that is, purely at the raid1 level, using just the data on its raid1 mirror(s). That means only a single device has to be read from in ordered to write the data to the rebuilding device. Raid0+1, by contrast, fails the entire raid0 level at once, thus requiring reading from an unfailed entire raid1 (higher) level mirror set while writing out an entire new raid0 set!! So while normal operation state is similar between raid10/raid1+0 and raid0+1, the recovery characteristics are **MUCH** different, with raid10 being markedly better than raid0+1. As a result, raid0+1 doesn't tend to be used that often in practice, while raid10 (aka raid1+0) has become quite common, particularly so as its performance is quite high, only exceeded by raid0, but with redundancy and recovery characteristics that are good to very good, as well. Its biggest negative at the low end is the number of devices required, normally a minimum of four (but see the Linux mdraid10 discussion below), a striped pair of mirrored pairs. This 1+0/0+1 distinction confused me as an early raid user for quite some time even after I knew the technical difference, as I kept trying to reverse them in my head, and I guess it confuses a lot of people. For some reason, my intuitive read of raid10 was the reverse of convention -- intuitively I /wanted/ to interpret it as a raid1 on top of raid0 instead of the raid0 on top of raid1 it is by convention, and even after I understood that there WAS a difference and in principle knew why and how, for years I actually had to look up the difference each time it came up, if it made a difference to the discussion, because I /wanted/ to read it backward, or more accurately, I thought the convention had it backward to the interpretation that made most sense to me. It is only recently that I came to see it the other way, and even still, I have to pause and think every time I see it, to ensure I'm not again reversing things. Which is the distinction that came up in the above discussion as well, only with raid5 and raid1 instead of raid0 and raid1. Apparently I'm not the only one to get things reversed! But yes, conceptually, raid15 is a raid5 layer on top of raid1, aka raid1 +5, while raid51 would be a raid1 layer on top of raid5, aka raid5+1. For the same recovery-time reasons noted above with raid0+1 vs. raid1+0/ raid10, having raid1 at the local/hardware layer should be preferable. With the basic concepts covered, the next level up is understanding that the Linux md/raid10 implementation, while BASED on the raid10 concept above, has some quite interesting extensions. Implementing it as a single software raid10 level instead of separate raid0 over raid1 allows some interesting optimizations and additional flexibility. Among other things, it no longer requires a minimum four devices (a raid0 pair of raid1 pairs) as separate raid0 over raid1 would. There's quite a bit of additional flexibility in layout. A detailed discussion is out of scope here, but googling raid10 on wikipedia is a good start, and the page it gives you actually discusses various other nested raid levels as well. >From there, follow the links to non-standard raid levels, and to the Linux mdraid implementation discussion, including the concepts of "near", "far", and "offset" layouts. https://en.wikipedia.org/wiki/Raid10 https://en.wikipedia.org/wiki/Non-standard_RAID_levels https://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10 But the discussion here is well beyond that, out toward further implementation detail and optimization. One of the problems that has been creeping up on us is the fact that as shear drive sizes increase, the possibility of undetected/uncorrected physical device errors goes up faster than the technology gets better at reducing them. For "simple" parity RAID solutions such as raid5, this is a rather big problem, because at some point, the chances of error during recovery scuttling the recovery entirely simply get too large to practically deal with, with recovery time (and thus time to recovery failure and try again) similarly increasing toward the days and weeks point. If recovery's going to take days, only for it to fail due to physical device error forcing another try... So the discussion is how to mitigate the problem. Multi-way-parity is of course the primary discussion in this thread, allowing detection and recovery of single-sector physical device errors via N-way-parity. But an integrated raid15 solution similar to mdraid's current raid10, is another possibility, effectively using the raid1 mirror level to mitigate sector-level physical device errors, while using the higher raid5 level to detect them and trigger a re-mirror at the raid1 level below it. But the only way that can work is if the two conceptually separate raid levels are integrated at the implementation level, so the raid5 level parity error detection can tell the raid1 level which of its mirrors is bad and force a remirroring from the good one(s) to the bad one. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
