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SubjectRE: 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
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.


> -----Original Message-----
> From: Alan Cox []
> Sent: Friday, June 08, 2001 5:37 PM
> To:
> Cc:;;
> 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
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