[Beowulf] Re: ECC support on motherboards?

Bill Broadley bill at cse.ucdavis.edu
Thu May 22 03:42:20 EDT 2008


Jim Lux wrote:
> Actually, not a big deal. The wearout is with erases/writes, not reads.  
> What they do is not use the same physical location for a given block.  
> That is, when you read/change/write a block back, it gets written to a 
> different location.   There's a systematic way to keep all this 
> straight, but the net result is that writes are evenly distributed 
> across the device.

Agreed, but only when they = SSD, CF, SDHC and the like.  I believe this does 
not apply to "flash".  The flash in your PDA, motherboard, cell phone, mp3 
player, etc does not have any innate load leveling in it.  Does it?

> Another significant feature of Flash RAM is that writes/erases are MUCH 
> slower than reads (even for the NAND flash that everyone uses these 
> days) But, still, blindingly fast compared to waiting for a disk to 
> rotate under the read/write head, so you can keep stuff in a cache, and 
> blast it out to the disk when you shutdown, before the power supply goes 
> away. (Most power supplies, for instance, can hold up for 8 
> milliseconds.. that's an eternity when your write time is measured in 
> nanoseconds)

Er, I've heard random write times in the 1/50th to 1/10th of a second
range, thus the reason people return expensive apple laptops with the $1k SSD 
option because outlook runs dramatically slower on their large mailboxes (a 
workloads that does lots of random tiny reads).  Amusingly executives with 
giant outlook mailboxes seem quite attracted to airbook SSD laptops.

Seems common for the SSDs to be 2-10 times slower at random writes and 1/3rd
of the bandwidth (read or write) of common cheap SATA desktop disks.  So sure 
on a device that expects mostly reads (MP3 players, light desktop use, etc.) 
it's quiet, reliable, and has better battery life.  But random writes or 
bandwidth aren't advantages... at least yet.

For these reasons it seems that for laptop/desktop use the main advantages are:
* Lower power
* smaller size
* higher vibration resistance
* faster random reads (read that boot faster)
* a low power buffer (so you spin up the disk less often)

I think vista and osx can even use flash specifically to accelerate booting 
with a normal USB connection flash device (no need to buy exotic disk drives 
with onboard flash that has been mentioned in the news numerous times).  I 
think there was even talk of one of the south bridges including on board flash 
for this purpose.
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