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From: Scott Long <scottl@samsco.org>
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To: Julian Elischer <julian@elischer.org>
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Cc: Attilio Rao <attilio@freebsd.org>, arch@freebsd.org
Subject: Re: sglist(9)
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On Nov 29, 2009, at 5:06 PM, Julian Elischer wrote:
> Scott Long wrote:
>
>> I think this is fundamentally wrong.  You're proposing exchanging a  
>> cheap operation of splitting VA's with an expensive operation of  
>> allocating, splitting, copying, and refcounting sglists.  Splitting  
>> is an excessively common operation, and your proposal will impact  
>> performance as storage becomes exponentially faster.
>
> From the perspective of a flashdrive driver the more
> efficient the better. The current generation of devices are
> doing 800MB/sec (6.4Gb/sec) of scattter-gather random IO
> and really that will only go up. We are doing over 130,000 independent
> transactions per second and we can put multiple drives in a single
> machine.
>
> These numbers will only increase with future developments.

MB/s doesn't tell me much other than the memory bandwidth of the  
pathways (and that that DMA engines involved don't completely suck).   
What about transactions/sec?  That tells me a lot more about the  
efficiency of the OS, drivers, and firmware, as well as latency.

Scott