Re: sd_mod or usb-storage fails to read a single good block (was: ehci_hcd fails to read a single good block)

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Wed, 2012-03-28 at 13:14 +0900, Norman Diamond wrote:
> I wrote:
> > James Bottomley corrected one of the experts who corrected me yesterday:
> >>>> So, the problem is that sd_mod is turning your request for a single block into a request for several blocks.
> >> 
> >> No, it won't be this.  Everything below block does exactly what block says.  If readahead is the problem, then you need to turn it off in block:
> >> echo 0 > /sys/block/<dev>/queue/read_ahead_kb
> > 
> > Thank you.  But...
> 
> But it didn't help.  Today I put the disk internally in an old notebook, and libata made it /dev/sda.
> echo 0 >/sys/block/sda/queue/read_ahead_kb
> but sg_dd still insisted on reading an entire page, so it refused to read a good block that was too close to the bad block.

This tells me you didn't read the man page on sg_dd.

To get it to read a block at a time using SCSI commands, you need

sg_dd blk_sgio=1 bs=512 bpt=1 <rest of the arguments>

That says use SG_IO and read exactly one block per SCSI command

James