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A SCSI Host adapter must also be separately configured into the system before a SCSI sequential access device can be configured.
It should be noted that tape devices are exclusive open devices, except in the case where a control mode device is opened. In the latter case, exclusive access is only sought when needed (e.g., to set parameters).
|00||A close will rewind the device; if the tape has been written, then a file mark will be written before the rewind is requested. The device is unmounted.|
|01||A close will leave the tape mounted. If the tape was written to, a file mark will be written. No other head positioning takes place. Any further reads or writes will occur directly after the last read, or the written file mark.|
|10||A close will rewind the device. If the tape has been written, then a file mark will be written before the rewind is requested. On completion of the rewind an unload command will be issued. The device is unmounted.|
Each write made to the device results in a single logical record
written to the tape.
One can never read or write
of a record from tape (though you may request a larger block and read
a smaller record); nor can one read multiple blocks.
Data from a single write is therefore read by a single read.
The block size used
may be any value supported by the device, the
adapter and the system (usually between 1 byte and 64 Kbytes,
When reading a variable record/block from the tape, the head is logically considered to be immediately after the last item read, and before the next item after that. If the next item is a file mark, but it was never read, then the next process to read will immediately hit the file mark and receive an end-of-file notification.
Data written by the user is passed to the tape as a succession of
fixed size blocks.
It may be contiguous in memory, but it is
considered to be a series of independent blocks.
One may never write
an amount of data that is not an exact multiple of the blocksize.
One may read and write the same data as a different set of records.
In other words, blocks that were written together may be read separately,
If one requests more blocks than remain in the file, the drive will encounter the file mark. As there is some data to return (unless there were no records before the file mark), the read will succeed, returning that data. The next read will return immediately with a value of 0. (As above, if the file mark is never read, it remains for the next process to read if in no-rewind mode.)
If the user would like his large reads and writes broken up into separate pieces, he may set the following loader tunables. Note that these tunables WILL GO AWAY in FreeBSD 11.0 . They are provided for transition purposes only.
This variable, when set to 1, will configure all sa devices to split large buffers into smaller pieces when needed.
This variable, when set to 1, will configure the given sa unit to split large buffers into multiple pieces. This will override the global setting, if it exists.
There are several sysctl(8) variables available to view block handling parameters:
This variable allows the user to see, but not modify, the current I/O split setting. The user is not permitted to modify this setting so that there is no chance of behavior changing for the application while a tape is mounted.
This variable shows the maximum I/O size in bytes that is allowed by the combination of kernel tuning parameters (MAXPHYS, DFLTPHYS) and the capabilities of the controller that is attached to the tape drive. Applications may look at this value for a guide on how large an I/O may be permitted, but should keep in mind that the actual maximum may be restricted further by the tape drive via the SCSI READ BLOCK LIMITS command.
This variable shows the maximum I/O size supported by the controller, in bytes, that is reported via the CAM Path Inquiry CCB (XPT_PATH_INQ). If this is 0, that means that the controller has not reported a maximum I/O size.
|sili||The default is 0. When set to 1, it sets the Suppress Incorrect Length Indicator (SILI) bit on tape reads. Tape drives normally return sense data (which contains the residual) when the application reads a block that is not the same length as the amount of data requested. The SILI bit suppresses that notification in most cases. See the SSC-5 spec (available at t10.org), specifically the section on the READ(6) command, for more information.|
|The default is 0. By default, the sa driver reports entering Programmable Early Warning, Early Warning and End of Media conditions by returning a write with 0 bytes written, and errno set to 0. If eot_warn is set to 1, the sa driver will set errno to ENOSPC when it enters any of the out of space conditions.|
|This is a read-only parameter, and is set to 1 if the tape drive supports protection information.|
|If protection is supported, set this to the desired protection method supported by the tape drive. As of SSC-5r03 (available at t10.org), the protection method values are:|
|1||Reed-Solomon CRC, 4 bytes in length.|
|2||CRC32C, 4 bytes in length.|
|Length of the protection information, see above for lengths.|
|If set to 1, enable logical block protection on writes. The CRC must be appended to the end of the block written to the tape driver. The tape drive will verify the CRC when it receives the block.|
|If set to 1, enable logical block protection on reads. The CRC will be appended to the end of the block read from the tape driver. The application should verify the CRC when it receives the block.|
|If set to 1, enable logical block protection on the RECOVER BUFFERED DATA command. The sa driver does not currently use the RECOVER BUFFERED DATA command.|
|/dev/sa0||Rewind on close|
|/dev/nsa0||No rewind on close|
|/dev/esa0||Eject on close (if capable)|
|/dev/sa0.ctl||Control mode device (to examine state while another program is accessing the device, e.g.).|
sysctl variable to 1. One EOM notification will be sent, BPEW status will be set for one position query, and then the driver state will be reset to normal.
The owner of record for many years was Matthew Jacob. The current maintainer is Kenneth Merry
Additionally, certain tapes (QIC tapes mostly) that were written under FreeBSD 2.X are not automatically read correctly with this driver: you may need to explicitly set variable block mode or set to the blocksize that works best for your device in order to read tapes written under FreeBSD 2.X.
Partitions are only supported for status information and location. It would be nice to add support for creating and editing tape partitions.
|SA (4)||May 5, 2017|
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|“||Computer science would have progressed much further and faster if all of the time and effort that has been spent maintaining and nurturing Unix had been spent on a sounder operating system.||”|
|— The Unix Haters' handbook|