On 02/13/16 11:49, Tinker wrote: > Hi, > > 1) > http://www.openbsd.org/papers/asiabsdcon2010_softraid/softraid.pdf page > 3 "2.2 RAID 1" says that it reads "on a round-robin basis from all > active chunks", i.e. read operations are spread evenly across disks. > > Since then did anyone implement selective reading based on experienced > read operation time, or a user-specified device read priority order? > > > That would allow Softraid RAID1 based on 1 SSD mirror + 1 SSD mirror + 1 > HDD mirror, which would give the best combination of IO performance and > data security OpenBSD would offer today.
I keep flip-flopping on the merits of this. At one point, I was with you, thinking, "great idea! Back an expensive, fast disk with a cheap disk". Currently, I'm thinking, "REALLY BAD IDEA". Here's my logic: There's no such thing as an "expensive disk" anymore. A quick look shows me that I can WALK INTO my local computer store and pick up a 2TB SSD for under $1000US. Now, that looks like a lot of money, and as a life-long cheapskate, when I get to four digits, I'm expecting at least two wheels and an engine. But in the Big Picture? No. That's one heck of a lot of stunningly fast storage for a reasonable chunk of change. Thirty-four years ago when I started in this business, I was installing 10MB disks for $2000/ea as fast as we could get the parts (and at that time, you could get a darned nice car for five of those drives, and a new Corvette cost less than ten of them). Now sure, the price has dropped a whole lot since then, and my first reaction would be "What does that have to do anything? I can buy 2TB disks for under $100, that's a huge savings over the SSD!" In raw dollars, sure. Percentage? Sure. In "value to business"? I don't think so. In 1982, people felt the computers of the day were worth adding $2000 to to get a tiny amount of "fast" storage to make their very few business apps run better. No question in their mind, it was worth it. Now we do much more with our computers and it costs much less. The business value of our investment should be much greater than it was in 1982. And ignoring hardware, it is. Companies drop thousands of dollars on consulting and assistance and think nothing of it. And in a major computer project, a couple $1000 disks barely show as a blip on the budget. Hey, I'm all about being a cheap bastard whenever possible, but this just isn't a reasonable place to be cheap, so not somewhere I'd suggest spending developer resources. Also ... it's probably a bad idea for functional reasons. You can't just assume that "slower" is better than "nothing" -- very often, it's indistinguishable from "nothing". In many cases, computer systems that perform below a certain speed are basically non-functional, as tasks can pile up on them faster than they can produce results. Anyone who has dealt with an overloaded database server, mail server or firewall will know what I'm saying here -- at a certain load, they go from "running ok" to "death spiral", and they do it very quickly. If you /need/ the speed of an SSD, you can justify the cost of a pair of 'em. If you can't justify the cost, you are really working with a really unimportant environment, and you can either wait for two cheap slow disks or skip the RAID entirely. How fast do you need to get to your porn, anyway? (now ... that being said, part of me would love a tmpfs / disk RAID1, one that would come up degraded, and the disk would populate the RAM disk, writes would go to both subsystems, reads would come from the RAM disk once populated. I could see this for some applications like CVS repositories or source directories where things are "read mostly", and typically smaller than a practical RAM size these days, and as there are still a few orders of magnitude greater performance in a RAM disk than an SSD and this will likely remain true for a while, there are SOME applications where this could be nice) > 2) > Also if there's a read/write failure (or excessive time consumption for > a single operation, say 15 seconds), will Softraid RAID1 learn to take > the broken disk out of use? As far as I am aware, Softraid (like most RAID systems, hw or sw) will deactivate a drive which reports a failure. Drives which go super slow (i.e., always manage to get the data BEFORE the X'th retry at which they would toss an error) never report an error back, so never deactivate the drive. Sound implausible? Nope. It Happens. Frustrating as heck when you have this happen to you until you figure it out. In fact, one key feature of "enterprise" and "RAID" grade disks is that when they hop off-line and throw an error fast and early, to prevent this problem (some "NAS" grade disks may do this. Or they may just see your credit limit hasn't been reached). However, having done this for a looong time, and seen the problems from both rapid-failure and "try and try" disks, I'll take the "try and try" problem any day. Happens a lot less often, and tends to be less catastrophic when it happens (hint: you WILL be quickly fixing a disk system which gets to be 100x slower than normal. You may not notice the first disk that fails and causes an array to be non-redundant until the disk fails that takes the array down completely). Nick.
