partition limits on scsi drives?

[Rick Stevens]

Jay Daniels wrote:
> What if any are the partition limits on scsi drives?
> 
> I have been setting up partition with linux fdisk, the first 3 being
> primary partitions and all others inside one big extended partition -
> commonly viewed with linux fdisk -l as  Win95 Ext'd (LBA).
> 
> In otherwords, I setup scsi drives the same as ide drives.
> 
> Do scsi drives have this same partition limit as ide drives or is this
> a limitation of the fdisk tool or system bios?

Essentially, LBA (and SCSI) make the disk look like a contiguous string
of "N" blocks, rather than IDE's native method of cylinder, head, sector
(or "CHS", a hold-over from the days of ST-506 or ST-412 MFM hard
drives).

The BIOS must be able to address all of the blocks.  In other words, a
32-bit unsigned integer can address 2^32 or 4,294,967,295 blocks (2TB).
BTW, this is the difference between Windows' FAT-12 (12-bit value,
2MB), FAT-16 (16-bit value, 32MB), and FAT-32 (32-bit value, 2TB)
filesystems.

Other limits are inherent in library routines such as lseek(2).  They
use a signed integer to seek around in a file.  A signed 32-bit integer
will only allow direct access to 2^31 bytes (2GB) of data, hence the
file size limit unless you have your libraries compiled with "LARGEFILE"
support (where you get a signed long, 2^63 bytes or a really, really
large number!).  Virtually all Linux stuff (well, gcc-compiled stuff)
use LARGEFILE, but you must make sure that the "offset" variable you
use is declared as

	off_t offset;

as opposed to the classic

	int offset;

which will stick you with the 2GB limit (remember, an "int" is the
"natural" size of the machine...32-bits on an x86).

> Furthermore, why is an extended partition listed in Linux as a
> "Win95" Ext'd (LBA)?

There's actually two types, type 5 (extended) and type f (win95 extended
LBA).  I'm not absolutely certain what the difference is other than
type f is a FAT-32 LBA-addressed partition.  I think type 5 is FAT-16
extended.  I could be wrong there.

And now you know far more than you ever cared to!  :-p
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
- Linux is like a wigwam...no windows, no gates...and apache inside! -
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