[ale] usb drives suck

[Jeff Hubbs]

Hmmm, the arbitration thing explains so much...now I can *explain* why I 
hate USB-connected HDDs.

On 1/25/10 8:03 PM, m-aaron-r wrote:
> On 2010/01/25, at 14:34 , Geoffrey wrote:
>
>    
>> Ed Cashin wrote:
>>      
>>> The iowait bucket is a subset of the old "idle" bucket.  It just
>>> means the CPU doesn't have much to do and you're doing I/O.
>>>
>>> Specifically, when the last thing that happened before the CPU
>>> went idle was to kick off some I/O, the tick gets counted in the
>>> iowait bucket.
>>>
>>> So there should be plenty of CPU for other processes while
>>> the system waits for the I/O to finish.  I don't know why the
>>> load average (running average count of processes that would
>>> like to run but have to wait) would go up unless there were lots
>>> of processes that couldn't run until the I/O finished.
>>>        
>> I don't get that either.  Right now, load average is 7.5, 6.49, 4.6
>> I'm running an rsync of /home to an external usb drive.
>>
>> I guarantee you, once it finishes, load average will drop below 1.
>> -- 
>> Until later, Geoffrey
>>      
> I had always wondered why drives connected (in daisy
> chains) on my Firewire (ieee-1394) buses delivered
> bi-directional (25mbps) DV video recording and playback
> across multiple drives so consistently, but the same external
> drive&  case connected as a dead end single device via a
> dedicated USB 2.0 connection  could not provide a
> continuous data stream and caused problems with video
> playback. It was especially confusing because, on paper,
> the USB 2.0 spec has a slightly higher data rate than 1394a
> (though only about half that of 1394b, which I also have on
> a couple of machines).
>
> Recently I found out the reason for the discrepancies and the
> video playback problems:  What the USB bus specs don't tell
> you is that USB data flow is SOFTWARE arbitrated, while
> Firewire (ieee-1394) buses are HARDWARE arbitrated.
>
> When the CPU is being heavily pressed into service decoding
> video streams and delivering frame accurate video and audio
> sync, I frequently see cases where the added interruptions of
> arbitrating the video data coming from USB bus will disrupt the
> media flow.
>
> I think the CPU load increases you are seeing when transferring
> large files are probably the result of software based USB bus
> arbitration making demands on the processor.
>
> peace
> aaron
>
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