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From: John McWilliams on 14 Apr 2010 14:09
Mark Conrad wrote:
> Besides, those solid-state drives have a nasty short-life
> expectency, when it comes to the number of write cycles
> before failure.
>
> They are quiet, though, and much less subject to being
> damaged by heat from extensive hard use.
>
> Too bad about the limited write cycles before failure,
> that is why I would never buy one at the present time.


Gosh 'n' golly, do you have a single cite of a site to back up either of
your statements?? (I am not counting the 'they are quiet' part.)

--
john mcwilliams


From: David Empson on 14 Apr 2010 20:52
Mark Conrad <aeiou(a)mostly.invalid> wrote:

> In article <1jgwi34.j4r0xbqem4imN%mikePOST(a)TOGROUPmacconsult.com>, Mike
> Rosenberg <mikePOST(a)TOGROUPmacconsult.com> wrote:
>
> > > Interesting tidbit from Apple, they claim that a 512 GB
> > > "solid ram drive" is available as an option, for a mere
> > > $1,300
> >
> > Um, are you using "claim" to suggest that you don't believe them?
>
> Yep, I definitely did not want to believe the $1,300 part.
>
> Besides, those solid-state drives have a nasty short-life
> expectency, when it comes to the number of write cycles
> before failure.

Do you really think Apple would be offering a 128 GB SSD as an option
for the primary boot drive in an Xserve (likely to be written very
often) if it was at risk of wearing out quickly?

Flash memory used in SSDs is rated for at least 100,000 write cycles to
one area of the flash memory.

SSDs use wear-levelling algorithms, which extend the useful life even
further. These work by avoiding frequent overwriting of a particular
area of the flash memory. If you have some heavily written part of the
file system (e.g. the bitmap keeping track of used space), the SSD will
use a different area of the flash memory each time that logical area of
the volume is rewritten, so the writes are spread throughout the flash
memory. The wearing out due to writing is averaged out over the entire
flash memory.

The end result is that SSDs have very good useful lifetime, except in
extreme situations.

See for example

http://www.memkor.com/en/technology/420hddvsssd.html

or more specifically (linked from the above):

http://www.memkor.com/en/technology/414we.html

This suggests a Flash SSD completely rewritten once per day will last at
least 250 years. (That's about 90000 write cycles.)

If less data is being written every day (e.g. typical frequent updates
of a small fraction of the drive), the SSD will last much longer due to
wear levelling.

With a 128 GB SSD, if you overwrote the whole thing (or an equivalent
total amount of data) 25 times per day it would still last in the order
of 10 years. That's 3.2 terabytes per day written to a 128 GB drive
(almost all of which would have been overwritten by later data).

As the drive capacity increases, that threshold would increase
proportionally: you'd need to be writing 12.8 terabytes per day to a 512
GB SSD to get its useful life down to 10 years.

An SSD may not be a good choice for a frequently overwritten and heavily
used huge database (unlikely to exist - databases usually accumulate
information rather than replacing it, especially in the volumes these
numbers suggest), but they are fine for "normal" computer use.

> They are quiet, though, and much less subject to being
> damaged by heat from extensive hard use.

Also much lower power consumption, which translates to more battery life
in a laptop.

Cost is the main reason that SSDs haven't completely replaced HDDs in
laptops. Write speed may be an issue for some high-end applications, but
SSDs can already achieve over 100 MB/s.

--
David Empson
dempson(a)actrix.gen.nz
From: JF Mezei on 15 Apr 2010 01:56
David Empson wrote:

> Do you really think Apple would be offering a 128 GB SSD as an option
> for the primary boot drive in an Xserve (likely to be written very
> often) if it was at risk of wearing out quickly?

Is this disk really flash based or RAM based (with backup battery) ? It
was my understanding that flash based disks don't have great
performance. Or is that an old wife's tale that is no longer valid with
flash disks now providing comparable performance to RAM based drives ?

From: David Empson on 15 Apr 2010 03:45
JF Mezei <jfmezei.spamnot(a)vaxination.ca> wrote:

> David Empson wrote:
>
> > Do you really think Apple would be offering a 128 GB SSD as an option
> > for the primary boot drive in an Xserve (likely to be written very
> > often) if it was at risk of wearing out quickly?
>
> Is this disk really flash based or RAM based (with backup battery) ?

Good point, but I can't find anything suggesting it is RAM with a
battery. That's old technology.

> It was my understanding that flash based disks don't have great
> performance. Or is that an old wife's tale that is no longer valid with
> flash disks now providing comparable performance to RAM based drives ?

This seems relevant:

<URL:http://www.apple.com/xserve/performance.html>

It shows roughly the same performance for the SSD and SATA (100 MB/s).

OWC has a chart showing SSD performance for a variety of brands (clearly
flash from points like "wear levelling" in the description), and most of
them can write faster than 200 MB/s.

<URL:http://eshop.macsales.com/item/Other%20World%20Computing/SSDMXE200/
>

--
David Empson
dempson(a)actrix.gen.nz
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