[Beowulf] SSD prices - q: how many writes/erases???

Jeff Layton laytonjb at att.net
Fri Dec 12 10:59:29 EST 2008


Peter Jakobi wrote:
> On Fri, Dec 12, 2008 at 08:17:19AM -0600, Geoff Jacobs wrote:
> Rehi,
>   
>>> Reliability is another question and I posted a quick response to
>>> this list in a different email.
>>>       
>> This being my big concern with flash.
>>     
>
> related is this topic on SSD / flashes:
>
> what's the life time when changing the same file frequently?
> aka "mapping block writes to cell erases" 
> aka "how  many  erases  are  possible?"
>   

This is somewhat a complicated question. It depends upon a
few factors if you are looking at things from the perspective
of the drive. In general the cells have a re-write limit that
is a function of what kind of cell it is. I don't remember
exact numbers, but I think MLC's are something like
10,000 rewrites and SLC's have like 100,000 rewrites.

But the wear-leveling algorithms do a reasonable job of
moving data to different cells rather than rewrite. This
"levels" out the number of rewrites to the cells.

What some people are doing to also help SSD's is to reserve
a portion of the drive as "backups" for cells that have
reached their limit. For example, you take a 64GB drive
and make it appear as a 50GB drive. Then the extra 14GB
is used by the drive to replace bad cells when needed (think
of it as the SSD approach that SATA drives have with spare
blocks that are used by the drive). While you lose space on
the drive, overall the drive can last longer because of the
spare cells. I think this is a good idea for MLC in particular
because of the low rewrite limit.

There should be some stuff floating around the web on the
topic of SSD's. Just treat some of the more "popular" stuff
from sites like Tom's Hardware, etc. with skepticism. :)

Jeff

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