hardware RAM disk

[Richard Pieri]

On May 12, 2009, at 2:51 PM, Jerry Feldman wrote:
> Could you elaborate. Right now I think the jury is still out on  
> these devices. Personally, back in then 1970s I was under the  
> impression that

Say that you are running some form of real-time (or near-real-time),  
high volume system.  The one I have experience is institutional  
equities trading.  A potential bottleneck is writing out the  
transaction logs.  Say that you need a minimum of 15,000 IOPS to keep  
up with your heaviest volume (I exaggerate by a factor of 10 for the  
sake of example).  What are your options?  The ones I had looked at are:

* Lots of rotating media striped together.  Given disks that can  
handle bursts of 300 IOPS I would need at least 50 such disks to  
handle the load.  Least expensive up front costs but requires the most  
power and cooling over a long term, especially if the application grows.

* Rotating media with lots of battery backed up cache.  This is  
cheaper on disk but in the case of EMC it actually costs a lot more up  
front than the disks.  Does not scale at all.  DMX frames have a  
fixed, finite, relatively small (IIRC, 128GB on DMX3) cache capacity.

* Flash-based SSDs.  These can handle the load but are quite  
expensive, but not as expensive as EMC cache.  Flash-based SSDs have  
the same wear problems as every other form of flash-based media.  Much  
of the expense is in redundant cells to offer MTBF comparable to  
rotating media.  Immune to power loss.

* SRAM or DRAM-based SSDs.  Fastest media, less expensive than flash  
and cache although more expensive than rotating media.  Longer MTBF  
than flash; no wear leveling needed.  More scaleable than cache.   
Susceptible to power failure if the batteries fail.

For high performance needs, where cache and rotating media are  
insufficient, there is a reasonable need for either flash or RAM  
SSDs.  Which is appropriate depends on the application.  For my  
example, based on a real project I worked on, I would use flash-based  
SSDs because I want the power fault tolerance.  For an application  
that requires the performance but not necessarily the fault tolerance  
I would consider RAM-based SSDs.

--Rich P.