micky wrote:
How it is possible that one SSD is 17 times as fast as another but costs
less? Both are 240G. Why would anyone buy the slower one (like I did
last summer)?

https://www.amazon.com/dp/B01N5IB20Q...t_details&th=1
350 Mb per second $35

https://www.amazon.com/PNY-CS900-240...01N5IB20Q?th=1
6 Gb per second $30 plus $6 for a bracket if you need one.


In the comparison list of the first one, 4 of them side by side, half
way to the bottom of the page, 2 others are the same speed as the first,
but the second one is 17 times as fast.

In a similar side-by-side comparison list on the second page, the same
thing is true. Only the PNY is so fast, and for less money. Does PNY
know something the others don't know.

Wire speed of interface:

6 gbit/sec * 8/10 * 1/8 = 600MB/sec (goodput rate)

8B10B bits per bytes
encode
DC Balance

The SATA connector with the seven pins, uses packets. The packets
are like "request" and "acknowledge". There are time gaps between
packets.

Using tagged queueing, more than one transfer can be in motion
at once. This makes it easier for the link to find something
to do.

But the usable rate with the packets, is about 500MB/sec or
somewhere in that ballpark. No packet protocol goes
exactly at 100.0% of link speed. There's always gaps of
various sizes. Ethernet packets have pretty small gaps between
them, but other things are quite bad at that, like USB2
(60MB/sec) runs mass storage protocol at 35MB/sec max.

There are a few SATA controller chips (add-ons) that don't
actually run at full speed either. There's a Marvell chip
with SATA III interface, where it can only send the packets
at ~300MB/sec. But things like Intel or AMD Southbridge SATA III
aren't like that, and they're not likely to be the bottleneck.
All of the hardware in those cases, can be capable of 500MB/sec
of packets.

*******

The Flash can only read and write, so fast. There is the page size
and the number of Flash channels. Modern drives have relatively
low chip counts. A small drive only has a couple flash chips plus
a nice controller. This compares to a USB stick which has a couple
flash chips and a crappy controller. (The SSD has ARM processor
cores inside its controller. The USB2 chip has an 8085 in it.
Comparatively speaking.)

The quality metric is write speed. Maybe a drive reads at 535MB/sec
(according to the label), but writes at 300MB/sec. If the drive
uses TLC or QLC based flash, it may have a section of Flash
that functions as a small cache (20GB perhaps). Any sustained
transfer, runs at the cache speed. But if a sustained transfer
(a Macrium backup .mrimg file maybe) exceeds 20GB in size, the
drive slows down on writes a bit.

A Samsung with "MLC-like" 3DNAND, the old ones really were MLC-like
and the drive did not use a cache, and had uniform speed from end
to end. Some of this years Samsung drives claim to be MLC-like
but they're not. They're using Flash cache like everyone else,
and Samsung falls off its pedestal. When they're forced to do that,
that's evidence the thing is just TLC-like and not worth a dime extra.

*******

Let's take my laptop as an example of how you could easily
get carried away with this stuff.

I buy a SATA III SSD at 500MB/sec, worried about getting
shafted on a purchase. Yet, if we examine my SATA port in
the laptop, it actually runs at SATA II speed, or half that
(250MB/sec max). OK, so I buy a "300MB/sec write" SSD.
See the problem ? The SSD is actually perfectly adequate for
the job and "keeps up with the rate". Whether I ran either
the 500MB/sec write SSD or the 300MB/sec write SSD, I
can't tell the difference, because the laptop only
does 250MB/sec on the wire.

You didn't need the fancy one after all :-)

And that's how my laptop goes. Gave it a SATA III SSD.
Runs at SATA II rates. What a waste, eh ?

Well, it's not entirely a waste. The main improvement
in SSD drives, is the reduction in seek time. It takes
no time to "move the heads". It doesn't matter how
files are scattered over the SSD surface. Takes the same
time to read them, no matter where they are. This is why
we buy the SSD.

Whether it writes at 300MB/sec or 250MB/sec, there could
be times when that speed helps. But not every second the
SSD is sitting there on the desktop, does it need to run
flat out like that. Lots of SSD operation is in tiny
bursts. It's not the end of the world if the sustained
isn't "the speed of light". Sure, you notice the speed
when you bench with HDTune. But for a lot of other
uses, it's not so bad after all.

All the SSDs are better than spinning rust, because
of the seek time. You know how Windows 10 likes to
"scan stuff for fun". That's a lot of seeks. And that's
why we have SSDs for Windows 10. Speed up the boot
might be the most impressive thing it does, all day long.
Hardly noticed otherwise.

Paul