[Beowulf] Checkpointing using flash
Lux, Jim (337C)
james.p.lux at jpl.nasa.gov
Fri Sep 21 09:29:31 PDT 2012
On 9/21/12 8:41 AM, "Hearns, John" <john.hearns at mclaren.com> wrote:
>Are your concerns about the accuracy of this statement related to the
>fact that elReg is claiming that they must dump "the entire memory" or
>some concern about flash being used as a temporary checkpointing medium?
>The "entire memory" statement puzzled me.
>But using flash in this fashion does seem a good idea.
Flash is slow, though... SLC NAND flash (pretty fast, 8 Gbit part) is 250
microseconds to write a 4kbyte (approx) page. Erasing is about 700
microseconds (reading is 25 microseconds)
MLC flash (say 512Gbit parts with 8 kBbyte pages) takes 1.3milliseconds to
write a page, 3.8 ms to erase (75us to read)... And has a life of 3000
That's 53 Mpbs streaming to the part. Yeah, any practical design is going
to have multiple interleaved devices, etc. so you can probably do it
But still, say you are checkpointing 8Gbyte.. That's 1300 seconds (yep,
about 20 minutes), assuming you've previously erased everything.
Even if you use the really fast SLC NAND, you're still looking at 500
You might be able to do it faster to a good hard disk with optimized data
Fast compared to disk, maybe, but very slow. Why not just mirror memory
(other than cost and power: RAM is much less dense than flash)
There's also the write cycle limit.. If you're looking for very high
densities (USB thumb drive) you're looking at
A) serial interfaces
B) MLC NAND with maybe 10k cycle life on each page
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