Re: NAND flash misery




"David Brown" <david@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:48676bc8$0$14992$8404b019@xxxxxxxxxxxxxxxxxx

HDDs wear out through use. The lifetime is roughly dependant on the
time the hard disk has been powered up, and how much the head is moved.

A HDD has one big problem: the lifetime tends to go to the end when it
falls on the floor.
Yes there are the special HDDs, accelerometers, suspensions and such;
however it makes HDD much less attractive then a flash alternatives.
Besides, a flash card is easy to swap.

It's thus fairly independent of the size.

But the small and light HDD is more likely to survive the contact with the
floor :)

Flash wears out through
erase-write cycles on blocks. So the more blocks you have to spread the
wear, the longer the lifetime, and the more you read rather than write,
the longer the lifetime.

The flash write endurance (as stated by the manufacturers) is at the order
of millions of cycles. This is more then enough for many applications; there
is really no point to bother about the wear leveling. The problem I am
facing is the infant mortality of the unreliable cells.

So as flash drives get bigger, they are
surpassing HDDs for reliability and lifetime. For common desktop usage,
a 32 GB flash disk will probably far outlast a typical hard disk - with
256 GB and bigger flash disks, even high quality hard disks are no
longer competitive on reliability and lifetime for real applications.

The big issue is the cost per GB - hard disks are still much much
cheaper. A second issue is speed - standard hard disks are
significantly faster than standard flash disks.

A common x300 8GB Lexar CF card from WallMart sustains the read/write speeds
at the order of 25...30MB/s. Those are the numbers that I measured; the
upper limit is because of the hardware, not the flash.

But that will change -
flash speeds are easily scaled (just use several devices in parallel)
once the price is right.

The falling on the floor, weight/size, power consumption, instant readyness
and the heat dissipation are the problems of HDDs which flash doesn't have.

Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com


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