On Wed, Oct 21, 2009 at 9:15 PM, Jake Caferilla <j...@tanooshka.com> wrote:
> Clearly a lot of people don't understand latency, so I'll talk about > latency, breaking it down in simpler components. > > Sometimes it helps to use made up numbers, to simplify a point. > > Imagine a non-real system that had these 'ridiculous' performance > characteristics: > > The system has a 60 second (1 minute) read latency. > The system can scale dramatically, it can do 60 billion IO's per minute. > > Now some here are arguing about the term latency, but its rather a simple > term. > It simply means the amount of time it takes, for data to move from one > point to another. > > And some here have argued there is no good measurement of latency, but also > it very simple. > It is measured in time units. > > OK, so we have a latency of 1 minute, in this 'explanatory' system. > > That means, I issued a read request, the Flash takes 1 minute to return the > data requested to the program. > > But remember, this example system, has massive parallel scalability. > > I issue 2 read requests, both read requests return after 1 minute. > I issue 3 read requests, all 3 return after 1 minute. > > I defined this made up system, as one, such that if you issue 60 billion > read requests, they all return, simultaneously, after 1 minute. > > Let's do some math. > > 60,000,000,000 divided by 60 seconds, well this system does 1 billion IOPS! > > Wow, what wouldn't run fast with 1 billion IOPS? > > The answer, is, most programs would not, not with such a high latency as > waiting 1 minute for data to return. Most apps wouldn't run acceptably, no > not at all. > > Imagine you are in Windows, or Solaris, or Linux, and every time you needed > to go to disk, a 1 minute wait. Wow, it would be totally unacceptable, > despite the IOPS, latency matters. > > Certain types of apps wouldn't be latency sensitive, some people would love > to have this 1 billion IOPs system :) > > The good news is, the F20 latency, even if we don flash has lower latency than traditional disks, that's part of what makes it > competitive...and by the same token, flash with lower latency than other > flash, has a competitive advantage. > > Some here say latency (that wait times) doesn't matter with flash. That > latency (waiting) only matters with traditional hard drives. > > Uhm, who told you that? I've never heard someone make that case before, > anywhere, ever. > > And lets give you credit and say you had some minor point to make about hdd > and flash differences...still you are using it in such a way, that someone > could draw the wrong conclusion, so..... clarify this point, you are > certainly not suggesting that higher wait times speeds up an application, > correct? > > Or that the F20's latency cannot impact performace, right? C'mon, some > common sense? anyone? > > Yet again, you're making up situations on paper. We're dealing with the real world, not theory. So please, describe the electronics that have been invented that can somehow take in 1billion IO requests, process them, have a memory back end that can return them, but does absolutely nothing with them for a full minute. Even if you scale those numbers down, your theory is absolutely ridiculous. Of course, you also failed to address the other issue. How exactly does a drive have .05ms response time, yet only provide 500 IOPS. It's IMPOSSIBLE for those numbers to work out. But hey, lets ignore reality and just go with vendor numbers. --Tim
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