Re: [tlug] Memory upgrade and CPU bit-width question

[Curt Sampson <cjs@example.com>]

On 2016-01-04 15:31 +0800 (Mon), Raymond Wan wrote:

> On an unrelated point to David's original query, I was looking at
> portable SSDs recently on a recent visit to Japan.  At Yodobashi
> Camera, they sold 512 GB ones at these prices:
> 
> 22990 Lexar
> 37180 Samsung
> 72130 Buffalo
> 
> How can Buffalo's be 3 times the price of Lexar?

My guess is that would be an indication that 512 GB drives are not yet
at the commodity pricing level, and so you're going to see quite a lot
of variation in the price.

Further support for this comes when you look at the $/GB ratio as
compared to say, 128 GB or 64 GB drives. It's clear where the curve is
there, and your best bet to save money is always to buy on the lowest
part of the $/quantity-of-whatever curve if you can.

On 2016-01-04 11:42 +0100 (Mon), Josh Glover wrote:

> By portable, do you mean USB? I was going to recommend ensuring that
> the USB bus could transfer data faster than a mechanical drive, but I
> just did a bit of research, and was astounded to see that USB 3.0 can
> transfer data at 5 Gbps! [1] The fastest mechanical drives seen to be
> capable of about 1.6 Gbps sustained read [2], so the bus won't be the
> bottleneck there. However, as USB 2.0 is limited to 480 Mbps, an SSD
> is pointless in that case.

Aw, Josh, I'd expected better of you! :-)

Even with a slow bus, an SSD is probably still quite pointful. Typical
read loads are not sequential but somewhat to very random, meaning
that the head on an HDD is going to be skipping around a lot, and then
waiting for the right sector to rotate around under the head. In most
workloads, the bus is utterly idle most of the time while waiting for
the head and disk do their physical movements that will allow them to
start reading the data.

This is why SSDs are so utterly brilliant (as you did correctly point
out elsewhere): they can have even slower data transfer rates than HDDs
and yet still, not having to move heads and platters around, they often
end up getting you the data faster than an HDD would. Let's visualize a
typical request:

       +- move heads around, wait wait wait for disk to spin
       |        found it! really fast transfer! ----+
       v                                            v
HDD:   |--------------------------------------------|===|
SDD:   |-----|=================|
       ^     ^                ^
       |     |                +-- oh, look how slow I was compared to the HDD!
       |     +-- found it! Now I start my *terribly* slow data transfer
       + --- start seeking
           
Note that in this to-scale diagram, the SSD transfered data six times
slower than the HDD.

> So, unless I'm missing something obvious, buy an external SSD only if
> you have a USB 3.0 port to connect it to....

Depends on the workload. It's true, there are a lot more workloads
typical of external drives (backups, for example) where fast random
access isn't as useful as it is on your main drive.

> My last ThinkPad had only one 3.0 port and three 2.0 ones, and it's a
> reasonably high-end model, so I don't think that all laptops have only
> 3.0 ports these days.

The 3.0 ports are easy to identify: the plastic bits in them are blue.
But keep in mind, even without USB 3.0, ESATA (and, if you want to get
really creepy old, "Firewire") gives you fast bulk transfer speeds as
well.

On 2016-01-05 23:52 +0800 (Tue), Raymond Wan wrote:

> Anyway, seems recent USB 3.0 SSDs are worth considering as a portable
> USB device.  I'm not quite ready to consider having it in a desktop as
> one of its internal drives, but maybe some day.

Josh is right that you've got that a**-backwards. Buy an SSD, make it
your main dive, and plop the now-somewhat-spare HDD in to an external
case for backup or whatever.

Certainly unless you have less than 4 GB of memory, and maybe even if
you have just 2 GB of memory, an SSD is going to to be by far the most
cost-effective way of increasing your computer's speed.

I have difficulty thinking of situations where someone's using an HDD
where my first (and potentially last) recommendation wouldn't be,
"install an SSD."

cjs
-- 
Curt Sampson         <cjs@example.com>         +81 90 7737 2974

To iterate is human, to recurse divine.
    - L Peter Deutsch