Linus Torvalds (torvalds@linux-foundation.org) on 8/21/08 wrote:
---------------------------
>Michael S (already5chosen@yahoo.com) on 8/21/08 wrote:
>>
>>Actually, nowadays TPC-C benchmarks use 73GB disks only
>>because 36GB disks becoming rare and cost about the same as
>>73GB.
>
>You miss the point.
>
>TPC-C doesn't use small disks because people want small
>disks.
>
>TPC-C uses small disks because with rotating media, you
>need a metric sh*tload of actuators to get the IOPS. So
>using big disks is a waste of everybody's time and money,
>because you still want about a million disks.
>
>But there are actually fairly high costs from using a
>million disks - and not just in money. There's a huge
>complexity cost in attaching that many disks, and it's
>only done because you have to. And it's actually seldom
>done in real life, because in any non-benchmark setting,
>the complexity cost simply isn't worth it!
>
>What SSD's could do is to give essentially the
>same number of IOPS, but using a much smaller set of disks.
>You could literally get the same number of IOPS using
>probably something like 20 SSD's instead of 2000 rotating
>disks!
>
>And if you only connect a small number of disks, it gives
>you a lot of other advantages. For example, you could now
>do it without any fancy fiberchannel setup or other exotic
>controllers. In fact, you could do it in a box that you
>actually expect people to use. Yes, it would still
>be expensive, but it wouldn't be insanely fragile like
>having several thousand disks connected is.
>
>In other words, you could have a realistic box that people
>would actually buy to run a benchmark on.
>
>And yes, I may be odd, but I think that the TPC rules
>really should say that the box you test is not only
>something you can buy, but that people actually
>do buy them in that configuration (with some minimum
>number of real sales).
>
>But realistically, you cannot do that right now, simply
>because of the SSD size issue. That will change,
>but it will take quite a few more years.
>
>So I agree - you could connect just a thousand SSD's, and
>you'd actually have enough disk. But connecting that many
>disks is what you want to get away from in the first place,
>and with that many disks you probably have more latency in
>the storage subsystem than you have in any of the disks!
>
>See what I'm trying to say? A thousand rotating disks is
>better than ten rotating disks, because it actually does
>improve performance through improving IOPS. But a thousand
>SSD's are actually likely to be inferior to ten
>SSD's because performance will probably not even go up!
>
>Yes, the IOPS may still scale, but it now scales at the
>cost of latency for any individual IO due to the much more
>complex interconnect, which means that it's no longer a
>good trade-off.
>
>Most people would take ten thousand IOPS with .1ms latency
>per IO over twice as many IOPS but with twice the latency
>per IO. The second version may get twice the throughput,
>but it needs four times as many IO's outstanding to get
>there!
>
>With rotating media, the latency of the more complex
>disk subsystem is not nearly as noticeable, because the
>per-IO latency is still dominated by the disk itself.
>
>So IOPS has been the limiting factor with rotating media,
>with individual IO latency being fairly flat (and even
>improving, because with bigger disks you can choose to only
>use a small portion of the disk an get somewhat better seek
>latencies).
>
>However, with SSD's an individual SSD can already
>do tons and tons of IOPS, and the limiting factor is more
>likely to be the individual IO latency and all the overhead
>to try to queue the IO in the first place.
>
>See?
>
>(And no, I have no numbers to back this up.)
>
>Linus

The tradition SSD (Solid State Disk) was battery backed DRAM. It has the single digit microsecond latency times, well over 100K IOPS limited only by the BW of the attaching channel. And write speeds are as fast as read speeds. They usually use low end ECC DRAM (read dirt cheap), but they are still quite expensive of over $50-100 per GB. Capacities are low (256-512GB) in a 5.25" FH form factor. There is also the hybrid SSD with either using flash or a regular HD to hold the data prior to the battery running out of power or an orderly shutdown. The contents are loaded back during the boot up sequence (although it can be done on an as needed basis too, if a slow start is ok).

This is used by real customers for RDBMS systems where performance is critical.

Pete