[Beowulf] SSD caching for parallel filesystems

Ellis H. Wilson III ellis at cse.psu.edu
Sun Feb 10 06:06:59 PST 2013


On 02/10/13 08:40, Vincent Diepeveen wrote:
>
> On Feb 10, 2013, at 2:09 PM, Ellis H. Wilson III wrote:
>
>> On 02/10/13 04:41, Vincent Diepeveen wrote:
>>> SSD's are not about bandwidth, they're about latency.
>>
>> This is a bit aggressive of a vantage point -- let's tone it back:
>> "SSD's aren't always the cheapest way to achieve bandwidth, but they are
>> critical for latency-sensitive applications that are too large for main
>> memory."
>
> SSD's are never the cheapest way to achieve bandwidth and never will be.

Ok.  These kinds of statements are my favorite of yours :D.

> Find me an application that needs big bandwidth and doesn't need massive
> storage.

Out-of-core ab-initio nuclear simulations.  Please see the paper I 
referenced a few emails ago.  These folks had a lot of data, certainly 
more than could fit in DRAM reasonably, but not so much that they needed 
tons and tons of HDDs to hold it.  Let's call it "medium-data."  Oddly 
enough, SSDs fall right in the "middle" between DRAM and HDD latencies 
and capacity.  Weird right?

> So any SSD solution that's *not* used for latency sensitive workloads,
> it needs thousands of
> dollars worth of SSD's.
> In such case plain old harddrive technology that's at buy in price right
> now $35 for a 2 TB disk
> (if you buy in a lot, that's the actual buy in price for big shops and
> you nor i get them for that price
> of course), or $17.5 a terabyte, that's unbeatable in performance for
> storage and bandwidth.

Right.  So since nobody I know can get them at that price, let's 
continue talking about them in terms of their real price: 150-200 
dollars.  I'm not going to have a discussion about imaginary pricing 
nobody I know of has access to.

> We speak about a sustained 200MB/s for dirt cheap RAID harddrives here.
> Put 16 of them in a raid partition and
> you can get more than you can deliver over the network from the file
> server and more than your motherboard can effectively
> handle a second.
>
> We speak about a buy in price of total peanuts for 16 harddrives here,
> and the same storage in SSD is
> worth a total fortune.

Please elaborate on total peanuts versus total fortune, and feel free to 
use real numbers from NewEgg or other retail figures.  Feel free to 
exclude eBay from your search.

> So using SSD's is just for latency. Anyone not using them for that i
> would never hire.

Oh well.  I was never much into chess anyhow, ;).

Best,

ellis


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