RE: question about scsi generic behavior

Well, the problem is not with non-512byte block device. The problem is
if the device is 512 byte block-sized device and if you use sg_dd with
bs=1024 or bs=2048, then it fails. I can understand if this fails
on a sequential device. But this fails on a disk. To be specific,
the problem is this : in case of fibre channel, you can specify
the FCP_DL in addition to the transfer-length which goes into CDB.
Now, when e.g. we use the following command,

sg_dd if=/dev/zero of=/dev/sg5 bs=2048 count=1,

and if in the low-level driver, we set the FCP_DL to 2048 and
in the CDB portition of FCP_CMND if we set the transfer-length to 1,
then the drive may not honour the FCP_DL and just look at the
transfer-length
and send XFER_RDY for 512 byte data. Once the HBA transfers the 512 byte of
data, then drive will send FCP_STATUS with status=good. Well, if you
look at it, we want to transfer 2048 bytes of data to the device, 
whereas the device completes the command with good status after 
transferring only 512 bytes. I hope this is more clear.

I guess, probably the sg driver is  probably not looking at the block 
size information returned in response to READ_CAPACITY command.

Regards,
-hiren
[phone]
[e-mail]

> -----Original Message-----
> From: Alan Cox [mailto:[e-mail]]
> Sent: Friday, June 08, 2001 5:37 PM
> To: [e-mail]
> Cc: [e-mail]; [e-mail];
> [e-mail]
> Subject: Re: question about scsi generic behavior
> 
> 
> > Hardcoding  of block size to 512 bytes for disk devices is 
> what currently 
> > either the block device driver or the sd driver is doing. 
> Because, if
> 
> I'm using 2048 byte block sized scsi media just fine. I've 
> not tried using
> sg on the same device
>