[mythtv-users] Fwd: Fast Forwarding Slow on USB hard drives

Yan-Fa Li yanfali at gmail.com
Sat Jul 28 00:34:26 UTC 2007

On 7/27/07, Steve Peters - Priority Electronics
<steve at priorityelectronics.com> wrote:
> Came out to 971.09 MB/sec for timing cached
> And for timing buffered disk reads, 25.11 mb/sec (now keep in mind that I
> was in the middle of transcoding 3 movies at the same time)...Also, keep in
> mind that the fast forwarding problem occurs when the system is running
> nothing but mythtv (ie, no transcoding going on).
> Also, last night when I hdparm'd it with the drive idle, it gave me 30mb/sec
> (internally it gives 75mb/sec, but 30 should be enough). The reason I can't
> put it internal is because I have already filled up the case (2 drives,
> small case), and also, the case is inside an enclosed cabinet, and it is
> nice for heat reasons to have the drive outside the cabinet.
> -steve

There's a lot of limitations in USB storage to do with the conversion
from SATA/UDMA operations into USB packets.  First thing USB storage
is implemented via SCSI.  So you drive is treated first as a SCSI
disk.  Block layer sends the data to the SCSI layer.  Then it gets
translated into USB packets.  Then it goes into the USB to UDMA/SATA
converter and gets translated back into the native drive operations.

This all adds latency and may not take advantage of the capabilities
of the drive like Native Command Queuing.  USB drives are my least
favorite external storage method, I really like eSATA because it's
basically a native interface.

I've also heard that USB operations are very CPU intensive because all
the heavy lifting is supposed to be done by the CPU, that's why it's
so cheap.  Take a look at your CPU usage while you run hdparm.  I'm
guessing it will be quite high.  My other pet peeve with USB storage
for servers is that you can't get any diagnostics from the drive.  I
like to run smartmon tools as it gives me a fighting chance in the
game of drive replacement chicken.


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