Are old SCSI tape drives not all created equal?
phil at ultimate.com
Thu Aug 18 10:20:28 CDT 2016
Jon Elson wrote:
> On 08/17/2016 01:07 PM, js at cimmeri.com wrote:
> > To my surprise, not all SCSI tape drives are created equal.
> Right, there was SCSI, SCSI-II and SCSI-III. Also, a lot of
> drives did not correctly support SOME features that others
> did. it got VERY messy. Other drives had weird timing
> restrictions, or certain commands must be given is a
> specific order, or they caused an error or lack of response.
To wit, from NetBSD's "scsi" man page:
Not every device obeys the SCSI specification as faithfully as it should.
As such devices are discovered by the NetBSD Project, their names are
added to a quirk list compiled into the scsi driver along a list of flags
indicating which particular bad behaviors the device exhibits (and that
the driver should be prepared to work around).
And in the "st" man page:
Because different tape drives behave differently, there is a mechanism
within the source to st to quickly and conveniently recognize and deal
with brands and models of drive that have special requirements.
There is a table (called the ``quirk table'') in which the identification
strings of known errant drives can be stored. Alongside each is a set of
flags that allows the setting of densities and blocksizes for each of the
modes, along with a set of `QUIRK' flags that can be used to enable or
disable sections of code within the driver if a particular drive is rec-
And the list of currently defined quirk bits:
#define ST_Q_FORCE_BLKSIZE 0x0001
#define ST_Q_SENSE_HELP 0x0002 /* must do READ for good MODE SENSE */
#define ST_Q_IGNORE_LOADS 0x0004
#define ST_Q_BLKSIZE 0x0008 /* variable-block media_blksize > 0 */
#define ST_Q_UNIMODAL 0x0010 /* unimode drive rejects mode select */
#define ST_Q_NOPREVENT 0x0020 /* does not support PREVENT */
#define ST_Q_ERASE_NOIMM 0x0040 /* drive rejects ERASE/w Immed bit */
#define ST_Q_NOFILEMARKS 0x0080 /* can only write 0 filemarks */
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