[sclug] RAID-1 on USB disks?

Tom Carbert-Allen tom at randominter.net
Tue Mar 10 10:22:09 UTC 2009


I think we made a big mistake here:
USB2 is 480megabits a second ~ 60 megabytes! Whoops, that was a school 
boy error...

I was able to get ~50mb/s from 8 drives but the downturn in performance 
after that was obviously because the bus was saturated above that point. 
So using more than one bus would be the only way forwards. I suppose 
this is also why the access time shot up, because the bus was stalling 
and wasting time. I also have no idea what overhead is introduced by 
having 32 cheap devices on the same bus. I don't know of/have experience 
with USB monitoring/debugging to resolve those questions.

So the answer is yes, even with cheap promo USB drives one can fully 
saturate a USB2 link to it's maximum capacity with good performance 
(although different types and numbers of drives will have greatly 
varying write speeds based on block size due to the whole "must write a 
whole X KB block" problem with flash)

But in your target machine it doesn't seem like performance is the goal, 
but do be aware that if you add more capacity and therefore accidentally 
go over the bus speed it won't degrade gracefully (at least not with the 
drives I tested) when you are reading from more than a a few of the 
sticks, eg RAID stripe or multiple reads from any other disk layout. So 
if you want more space you will have to buy SLOWER drives or limit 
access speed as not to hit the bus ceiling as it doesn't seem like your 
target devices is capable of supporting more than one bus. Even cheap 
modern 4gb sticks give out over 25mbs nower days, so you are limited to 
a pair.

If you are looking for a cheap low power linux system that can take lots 
of IO space and speed, there are many other options, although not quite 
as small as the shavaplug thingy. I have used in the past: second hand 
modded p3 laptop=11w, mini itx eden board=9w, linksys router with 
USB2=not sure but probably <5w all of which were <$100


David Given wrote:
> Hash: SHA1
> Tom Carbert-Allen wrote:
> [...]
>> I tried this after a trade show when some nice company left a box full
>> or promotional USB drives behind next to the rubbish....
>> Problem was, the more I added the worse the performance got not better
>> after the first few. Access times shot up, CPU usage shot up even more.
> Hmm. That's interesting --- the (few) sources I've found show it works
> reasonably well (50MB/s read, 30MB/s write, 1ms seek):
> http://linuxgazette.net/151/weiner.html
> I know that USB is fairly CPU-intensive, but even on an ultra-low-power
> 266MHz ARM (an NSLU2) it was maxing out at 5% or so.
> What sort of drives were they? Promotional hardware I've seen before is
> likely to be old and cheap, and therefore probably a bit dodgy.
> [...]
>> I wish I had more than two machines to test it on (Mac mini dual core
>> running cent os and laptop running ubuntu)
>> I wonder if it would be faster if I inserted a PCI usb card
> One would have thought that Apple hardware should have a reasonable USB
> chipset... I don't know whether using a PCI USB adaptor would have
> helped; you'd spread the load across different USB channels but you also
> have the CPU having to do more management. I wouldn't have thought that
> mass storage bandwidths would stress a 480MB/s USB 2 system, anyway.
> [...]
>> The OCZ solid disks which just poped up for under 70 quid on the other
>> hand.... ARE fast. I now have 8 of them and am a very happy man.
> Alas, they're over twice the price of thumb drives (2.6 UKP/GB as
> opposed to about 1.1). Also, my hypothetical target machine doesn't have
> SATA. Or PATA, or PCI for that matter. It's a Sheevaplug; 1.2GHz ARM,
> 512MB RAM, 512MB flash, 5W, $100, the size of a power brick. The only
> downside is that it's got one (1) SDIO card reader slot, one (1) USB 2
> slot, and one (1) Ethernet port. Plus a serial console.
> http://www.linuxdevices.com/news/NS9634061300.html
> - --
> David Given
> dg at cowlark.com
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> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
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