Logical vs. physical blocks on various media and how to detect
Kevin D Butt
kdbutt at us.ibm.com
Tue Oct 11 11:06:32 PDT 2011
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My response relates to tape devices. According to SSC-4r02 clause
18.104.22.168, "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
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
Data Protection & Retention
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at us.ibm.com
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
Sent by: owner-t10 at t10.org
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
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
account for every type media, such as for CD, DVD and BD ?
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