[Tfug] Crappy USB LVM/Software RAID10 performance?

[Zack Williams]

> I just bought two 750GB external USB drives to replace the internal 2x120GB
> hardware RAID1 setup I have here at the house.  Each drive is plugged into a
> seperate USB hub on my box. I set up the drives in a RAID10 relationship to
> make them as fast as possible. From there, I created a volume group on the
> resulting device, built an logical volume within it spanning the whole
> drive, formatted it as ext3 and mounted it. No problems anywhere along the
> line, no hiccups, nothing.

Separate USB card per drive (so each gets it's own controller) or all
into one onboard USB controller?

You have 4 drives?  RAID 10 requires that at minimum.

> USB 2.0 is spec'ed @ 480Mb/sec max, which means the pipe should be able to
> handle upwards of ~60MB/sec under ideal circumstances. These are probably
> SATA drives i'm working with, ones easilly capable of 35-40MB/sec regardless
> of method. Even if they were IDE drives, i'd expect better performance,
> upwards of 25-30MB/sec.

Don't trust the marketing specs - USB 2.0 really tops out at about 33MB/sec.

From: http://arstechnica.com/hardware/news/2009/05/usb-flash-drive-roundup.ars/3

"Secondly, USB2.0 is reaching the end of the road. While the raw
maximum throughput of USB2 is 480Mbps (which translates to around
53MB/sec), the actual maximum throughput, after factoring in overhead
for the protocol and signaling limitations, puts the practical limit
around 33MB/sec. This severely limits the potential of modern flash
memory with respect to its maximum read speeds and will soon put a
damper on sequential write speeds in the near future."

> So where's the crappy performance stemming from?

I'm going to guess either a slow USB controller, a slow drive
bridgeboard, or contention among devices on the bus.

In the future, go with a more storage oriented tech like Firewire,
SCSI or eSATA.

- Zack