Re: [PATCH 2/3] SCSI core and sd: early detection of medium notpresent

From: Alan Stern
Date: Fri Jun 09 2006 - 10:54:02 EST


On Fri, 9 Jun 2006, James Bottomley wrote:

> > Ultimately this boils down to how you want to represent "No medium
> > present" in the SCSI core. What do you think is the bets way?
>
> Well ... that's where I think we follow the CD people, since they're the
> ones who have this occurring the most often.

Okay, I'll wait for Jens to take a look and make a recommendation.

BTW, the stimulus for this work came from a user with a USB flash card
reader. The reader implements 4 different LUNs (for the different sorts
of flash technology) and hald polls each LUN every 2 seconds. Lack of
early detection for no-medium-present means that each poll ends up
generating 8 SCSI commands; hence 16 commands get sent per second. Not
only is this aesthetically displeasing, on this user's system some
low-quality USB hardware was chewing up a high percentage of the total PCI
bandwidth because of the frequent requests. Poorly-selected timeouts in
the USB host controller driver contributed to the problem as well. The
final result was that on an unloaded system doing nothing at all, the CPU
load was nevertheless up to 40%!

Changing hardware and updating the USB host driver helped the user, but I
still think it's a good idea to eliminate the useless commands.

Alan Stern

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/