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On Mon, 26 May 2008, Mertol Ozyoney wrote:
> It's true that NAND based falsh's wear out under heavy load. Regular
> consumer grade nand drives will wear out the extra cells pretty rapidly. (in
> a year or so) However enterprise grade SSD disks are fine tuned to with
> stand continous writes for more than 10 years
It is incorrect to classify wear in terms of years without also
specifying update behavior. NAND FLASH sectors can withstand 100,000
to (sometimes) 1,000,000 write-erase-cycles. In normal filesystem
use, there are far more reads than writes and the size of the storage
device is much larger than the the data re-written. Even in server
use, only a small fraction of the data is updated. A device used to
cache writes will be written to as often as it is read from (or
perhaps more often). If the cache device storage is fully occupied,
then wear leveling algorithms based on statistics do not have much
opportunity to work.
If the underlying device sectors are good for 100,000
write-erase-cycles and the entire device is re-written once per
second, then the device is not going to last very long (27 hours).
Of course the write performance for these devices is quite poor
(8-120MB/second) and the write performace seems to be proportional to
the total storage size so it is quite unlikely that you could re-write
a suitably performant device once per second. The performance of
FLASH SSDs does not seem very appropriate for use as a write cache
device.
There is a useful guide to these devices at
"http://www.storagesearch.com/ssd-buyers-guide.html";.
SRAM-based cache devices which plug into a PCI-X or PCI-Express slot
seem far more appropriate for use as a write cache than a slow SATA
device. At least 5X or 10X the performance is available by this
means.
Bob
======================================
Bob Friesenhahn
bfriesen@xxxxxxxxxxxxxxxxxxx, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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