Logical vs. physical blocks on various media and how to detect

Gerry Houlder gerry.houlder at seagate.com
Tue Oct 11 12:51:16 PDT 2011

Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1110112_f.htm">HTML-formatted message</a>

For direct access devices, there is a field in the READ CAPACITY (16)
command that returns the "number of logical blocks per physical block
exponent". If this field indicates that there is more than one logical block
per physical block, then you will see the same behavior that you described
for DVDs and CDs. Most devices are still mapped 1 to 1 but next generation
products are likely to use multiple logical blocks per physical block
Some existing SSD/ Thumb drive products actually put multiple LBs in the
same "physical block" but they still apply ECC correction on a "per logical
block" basis, so they still qualify as 1 to 1 mapping.
On Tue, Oct 11, 2011 at 1:06 PM, Kevin D Butt <kdbutt at us.ibm.com> wrote:
> My response relates to tape devices.	According to SSC-4r02 clause,
> "The basic unit of data transferred by an application client is called a
> logical block. Logical blocks are stored according to the specifications of
> the format for the volume and may be recorded as portions of one or more
> physical blocks on the medium. The mapping between physical and logical
> blocks is the responsibility of the device server."
> While not explicitly stating it, each tape vendor essentially determines
> what the mapping is and it is vendor specific.  But also, since tape
> supports variable length blocks and it is its primary mode of operation,
> each physical block contains x number of bytes (as determined by the
> vendor-specific format) and each logical block is mapped into one or more
> physical blocks.
> Regards,
> Kevin D. Butt
> SCSI & Fibre Channel Architect, Tape Firmware
> Data Protection & Retention
> MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
> Tel: 520-799-5280
> Fax: 520-799-2723 (T/L:321)
> Email address: kdbutt at us.ibm.com
> http://www-03.ibm.com/servers/storage/
> From:        "Peter Van Hove" <peter at Smart-Projects.net>
> To:	     "T10 Reflector" <t10 at t10.org>
> Date:        10/11/2011 10:40 AM
> Subject:	  Logical vs. physical blocks on various media and how to
> detect
> Sent by:	  owner-t10 at t10.org
> ------------------------------
> Hi,
> I know that on CD, a logical block is also a physical block.	The error
> detection and correction is done on that one block and if block x is
> unreadable, blocks x-1 and x+1 may very well still be readable.
> On DVD it is different.  The logical block size is 2K but an actual
> (physical) ECC block spans 16 of those logical blocks.  Error detection and
> correction is applied to the 32K ECC block rather than every logical 2K
> block.
> If one logical 2K block is unreadable, then infact 16 of those are
> unreadable as the entire ECC block is unreadable.
> On Blu ray finally, the same principle but then 32 logical 2K blocks or a
> physical block of 64 K.
> I was wondering if I can apply the same logic on thumbdrives / SD cards,
> SSM, Hard Drives etc.
> I don't know however, if the same logic applies, how to get that
> information from a device, or is there a fixed number to be taken in
> for every type media, such as for CD, DVD and BD ?

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