[ltp] Re: solid state drive?

[Theodore Tso]

On Wed, Feb 18, 2009 at 07:34:33AM +0100, Marius Gedminas wrote:
> On Wed, Feb 18, 2009 at 12:24:10AM +0100, Marius Gedminas wrote:
> > This is a very informative and interesting post, thanks!
> > 
> > On Mon, Feb 16, 2009 at 08:13:30PM -0500, Theodore Tso wrote:
> > > As far as prices are concerned, the Intel X25-M SSD is more expensive
> > > than traditional MLC drives, but cheaper than SLC drives.  The street
> > > price of the 80GB SSD is $400.  Sure, that's about 4x the price of an
> > > HDD, but given its shock resistance and speed, that's not bad.  In
> > > contrast, traditional 64GB SLC drives cost about $500, and traditional
> > > 64GB MLC drives cost about $140.  (And an 80GB HDD is about $40).
> > 
> > This is slightly confusing: how is a $400 SSD about 4x the price of a
> > $40 HDD?

Sorry, you're right, it's about 10x an equivalently sized HDD.  I
didn't realize how much 80GB HDD's had fallen in price until I had
checked.  These days, I tend to buy 300GB+ laptop drives.  :-)

> Also, how do Compact Flash cards fit in the SLC/MLC hierarchy?  They're
> also solid-state storage devices masquerading as (ATA) hard disks,
> right?

CF manufacturers generally don't you a lot about what's inside their
cards, so it varies a lot.  You can bet the cheaper ones are MLC; in
general it's probably better to assume MLC unless the manufacturer
explicitly states otherwise.  The primary problem with CF is that they
are limited by their ATA interface.  If the camera (and the card)
doesn't do UDMA, they are limited to about 16MB/s.  If the camera (and
the card) does UDMA, they could in theory go up to 133 MB/s, but I'm
not sure I've seen camera (and cards) that do more than 100 MB/s (UDMA
mode 5).  Some cards may only do UDMA mode 3 (45 MB/s).  

SATA speeds let you go at up to 150 Mb/s and 300 MB/s, which for flash
is more important when you are reading as opposed to writing.  For
example the Intel X25-E has a maximum read bandwidth of 250 MB/s, and
a maximum write bandwidth of 170 MB/s.  The X25-M has a maximum read
bandwidth of 250 MB/s, and a maximum write bandwidth of 70 MB/s.
(This is because the X25-M uses MLC, and the X25-E uses SLC; the Intel
X25 design avoids the write amplification problem, but if you are
doing streaming writes, you will still be limited by maximum write
speeds of the flash.)  For contrast, a modern 5400rpm laptop HDD has
write speeds of around 55 Mb/s, and a 7200rpm HDD has write speeds of
around 90 Mb/s.

						- Ted