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

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James Bottomley wrote:
> 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.

Guilty as charged.  Sorry, for some reason I didn't imagine that its parameters would differ from 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

Thank you.

I don't want to manhandle that old notebook's fragile vendor proprietary IDE cable again, but I'll try to find another way to test that drive again without going though USB.
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