big SCSI == LOST_DATA????

[Poster of News]

In article <1991Mar13.153825.22240 at banana.fedex.com> bill at banana.fedex.com (bill daniels) writes:
>An front-page article in the March 4, 1991, issue of _Digital News_ deals
>with problems associated with using large (> 1.2G) SCSI disks under Ultrix
>(through 4.1) and VMS.  The reason given is adherence to SCSI Group 0 commands.
>Supposedly, the group 0 commands break down when referencing large addresses.
>My question is:  Does this address wraparound problem arise any time one
>addresses the last portions of a disk OR only if the disk is considered
>one large (read Ultrix "c") partition?  Will the same problem appear if the
>latter part of the drive is a smaller "h" partition?

The problem is real and does NOT depend on the file system section number.
The driver in many systems cannot (by using type 0 commands) access
block numbers that require more than 21 bits = ~1.2Gbytes.

File system sections are basically offsets added to the desired block number:
for example, if section H starts at block 10000, asking for block 3
of section H is mapped by the operating system to 10000 + 3.  The
disk drive only sees a request for block 10003, it doesn't know of sections.
So in your example, any reference to that section H will clobber your
drive's low block numbers.

BTW: Since the address in the type 0 commands is a block address,
not a byte address, the problem will appear later (i.e. with bigger
drives) on systems that format disks with larger block sizes (i.e., 1024)
But in any case, the right solution is to have the kernel generate
the larger command when appropriate.

Marc Pucci
marc at bellcore.com

PS:  There is yet another problem waiting in the wings for unix user's
since the read/write seek pointer is limited to 31 bits, it will
soon be impossible to make random access to very large disks.