Large disks hazardous to your data?
Patrick L. Nolan
pln at egret1.stanford.edu
Wed Mar 20 04:33:00 AEST 1991
The March 4 issue of Digital News has an article with the headline "User
data at risk with 1.2 GB disks." Here are some excerpts from the article:
Customers configuring ... SCSI hard disk drives of more than 1 GB run
a serious risk of losing their stored data.
The SCSI implementation found in Ultrix and all VMS operating
systems except version 5.4 is unable to address hard disk drives
larger than 1 Gbyte, meaning that users with disks formatted at
more than 1 Gbyte are paying for storage space they cannot possibly
access. And, in attempting to do so, they could lose all their data.
The problem ... does not occur when using a later version of the
SCSI I/O driver, called Extended SCSI.
According to SCSI specialists, the SCSI Group 0 set of commands,
which are also embedded into some Sun Microsystems ... operating
systems, can address a maximum of 1.073742 GBytes. On disks
with a larger formatted capacity, the SCSI I/O driver wraps
around the disk and overwrites the first block of data, called
the superblock or home block, thereby preventing access to the rest of
the data on the disk....
"The Group 0 commands are a relatively primitive SCSI I/O driver;
they only allocate 6 bytes for addressing and don't offer
diagnostics ... Most of the new SCSI implementations use the
Group 1 or Extended SCSI commands."
This is pretty alarming. I think it would be of general interest if some
experts on the Sun SCSI implementation could tell us whether we have Group
0 or Group 1. Are they implemented in hardware or software?
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