Any issues with 4k block size on SCSI Disks?

John Lohmeyer lohmeyer at
Tue Oct 9 11:33:58 PDT 2001

* From the T10 Reflector (t10 at, posted by:
* John Lohmeyer <lohmeyer at>
I promised to summarize the responses I received on the question about 4k 
block size issues with SCSI.

Aaron Dailey <aaron.dailey at> reported:

>We've done some work with non standard block sizes (although not 4k), and I
>believe the only issue we've seen is that it's not clear how to reformat a
>disk to non standard block sizes. Apparently IBM, Seagate and Fujitsu all
>have different methods.

I responded:

>The original intention for changing the formatted block size was that the
>Application Client would use the MODE SELECT command to set the desired block
>length and the FORMAT UNIT command to make it happen. From the SCSI-2
>direct-access device model:
>"The number of bytes of data contained in a logical block is known as the
>block length. Each logical block has a block length associated with it.
>The block length may be different for each logical block on the medium.
>However, in a typical device only one block length is used at a time. The
>block descriptor in the MODE SENSE data describes the block lengths that
>are used on the medium. A MODE SELECT command can be used to set up
>extents. An extent is a specified number of logical blocks that have the
>specified block length. The FORMAT UNIT command is typically required to
>change the block length of devices that support variable block lengths and
>make the extents that were setup active."

He followed up with:

>Some details. In addition to what you quote from the SCSI 2 spec, one of the
>vendors spec'd that we needed to change DATA BYTES PER PHYSICAL SECTOR in the
>FORMAT DEVICE mode page. I think there was also some disagreement about the
>mode select - could it also specify a mode page, or
>should it only contain the mode header.

I have not received any other responses.


John Lohmeyer                  Email: lohmeyer at
LSI Logic Corp.                Voice: +1-719-533-7560
4420 ArrowsWest Dr.              Fax: +1-719-533-7183
Colo Spgs, CO 80907

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