Documentation/ide/ide-tape.txt - kernel/common - Git at Google

 /*
  * IDE ATAPI streaming tape driver.
  *
  * This driver is a part of the Linux ide driver.
  *
  * The driver, in co-operation with ide.c, basically traverses the
  * request-list for the block device interface. The character device
  * interface, on the other hand, creates new requests, adds them
  * to the request-list of the block device, and waits for their completion.
  *
  * Pipelined operation mode is now supported on both reads and writes.
  *
  * The block device major and minor numbers are determined from the
  * tape's relative position in the ide interfaces, as explained in ide.c.
  *
  * The character device interface consists of the following devices:
  *
  * ht0		major 37, minor 0	first  IDE tape, rewind on close.
  * ht1		major 37, minor 1	second IDE tape, rewind on close.
  * ...
  * nht0		major 37, minor 128	first  IDE tape, no rewind on close.
  * nht1		major 37, minor 129	second IDE tape, no rewind on close.
  * ...
  *
  * The general magnetic tape commands compatible interface, as defined by
  * include/linux/mtio.h, is accessible through the character device.
  *
  * General ide driver configuration options, such as the interrupt-unmask
  * flag, can be configured by issuing an ioctl to the block device interface,
  * as any other ide device.
  *
  * Our own ide-tape ioctl's can be issued to either the block device or
  * the character device interface.
  *
  * Maximal throughput with minimal bus load will usually be achieved in the
  * following scenario:
  *
  *	1.	ide-tape is operating in the pipelined operation mode.
  *	2.	No buffering is performed by the user backup program.
  *
  * Testing was done with a 2 GB CONNER CTMA 4000 IDE ATAPI Streaming Tape Drive.
  *
  * Here are some words from the first releases of hd.c, which are quoted
  * in ide.c and apply here as well:
  *
  * | Special care is recommended.  Have Fun!
  *
  *
  * An overview of the pipelined operation mode.
  *
  * In the pipelined write mode, we will usually just add requests to our
  * pipeline and return immediately, before we even start to service them. The
  * user program will then have enough time to prepare the next request while
  * we are still busy servicing previous requests. In the pipelined read mode,
  * the situation is similar - we add read-ahead requests into the pipeline,
  * before the user even requested them.
  *
  * The pipeline can be viewed as a "safety net" which will be activated when
  * the system load is high and prevents the user backup program from keeping up
  * with the current tape speed. At this point, the pipeline will get
  * shorter and shorter but the tape will still be streaming at the same speed.
  * Assuming we have enough pipeline stages, the system load will hopefully
  * decrease before the pipeline is completely empty, and the backup program
  * will be able to "catch up" and refill the pipeline again.
  *
  * When using the pipelined mode, it would be best to disable any type of
  * buffering done by the user program, as ide-tape already provides all the
  * benefits in the kernel, where it can be done in a more efficient way.
  * As we will usually not block the user program on a request, the most
  * efficient user code will then be a simple read-write-read-... cycle.
  * Any additional logic will usually just slow down the backup process.
  *
  * Using the pipelined mode, I get a constant over 400 KBps throughput,
  * which seems to be the maximum throughput supported by my tape.
  *
  * However, there are some downfalls:
  *
  *	1.	We use memory (for data buffers) in proportional to the number
  *		of pipeline stages (each stage is about 26 KB with my tape).
  *	2.	In the pipelined write mode, we cheat and postpone error codes
  *		to the user task. In read mode, the actual tape position
  *		will be a bit further than the last requested block.
  *
  * Concerning (1):
  *
  *	1.	We allocate stages dynamically only when we need them. When
  *		we don't need them, we don't consume additional memory. In
  *		case we can't allocate stages, we just manage without them
  *		(at the expense of decreased throughput) so when Linux is
  *		tight in memory, we will not pose additional difficulties.
  *
  *	2.	The maximum number of stages (which is, in fact, the maximum
  *		amount of memory) which we allocate is limited by the compile
  *		time parameter IDETAPE_MAX_PIPELINE_STAGES.
  *
  *	3.	The maximum number of stages is a controlled parameter - We
  *		don't start from the user defined maximum number of stages
  *		but from the lower IDETAPE_MIN_PIPELINE_STAGES (again, we
  *		will not even allocate this amount of stages if the user
  *		program can't handle the speed). We then implement a feedback
  *		loop which checks if the pipeline is empty, and if it is, we
  *		increase the maximum number of stages as necessary until we
  *		reach the optimum value which just manages to keep the tape
  *		busy with minimum allocated memory or until we reach
  *		IDETAPE_MAX_PIPELINE_STAGES.
  *
  * Concerning (2):
  *
  *	In pipelined write mode, ide-tape can not return accurate error codes
  *	to the user program since we usually just add the request to the
  *      pipeline without waiting for it to be serviced. In case an error
  *      occurs, I will report it on the next user request.
  *
  *	In the pipelined read mode, subsequent read requests or forward
  *	filemark spacing will perform correctly, as we preserve all blocks
  *	and filemarks which we encountered during our excess read-ahead.
  *
  *	For accurate tape positioning and error reporting, disabling
  *	pipelined mode might be the best option.
  *
  * You can enable/disable/tune the pipelined operation mode by adjusting
  * the compile time parameters below.
  *
  *
  *	Possible improvements.
  *
  *	1.	Support for the ATAPI overlap protocol.
  *
  *		In order to maximize bus throughput, we currently use the DSC
  *		overlap method which enables ide.c to service requests from the
  *		other device while the tape is busy executing a command. The
  *		DSC overlap method involves polling the tape's status register
  *		for the DSC bit, and servicing the other device while the tape
  *		isn't ready.
  *
  *		In the current QIC development standard (December 1995),
  *		it is recommended that new tape drives will *in addition*
  *		implement the ATAPI overlap protocol, which is used for the
  *		same purpose - efficient use of the IDE bus, but is interrupt
  *		driven and thus has much less CPU overhead.
  *
  *		ATAPI overlap is likely to be supported in most new ATAPI
  *		devices, including new ATAPI cdroms, and thus provides us
  *		a method by which we can achieve higher throughput when
  *		sharing a (fast) ATA-2 disk with any (slow) new ATAPI device.
  */
	/*
	* IDE ATAPI streaming tape driver.
	*
	* This driver is a part of the Linux ide driver.
	*
	* The driver, in co-operation with ide.c, basically traverses the
	* request-list for the block device interface. The character device
	* interface, on the other hand, creates new requests, adds them
	* to the request-list of the block device, and waits for their completion.
	*
	* Pipelined operation mode is now supported on both reads and writes.
	*
	* The block device major and minor numbers are determined from the
	* tape's relative position in the ide interfaces, as explained in ide.c.
	*
	* The character device interface consists of the following devices:
	*
	* ht0 major 37, minor 0 first IDE tape, rewind on close.
	* ht1 major 37, minor 1 second IDE tape, rewind on close.
	* ...
	* nht0 major 37, minor 128 first IDE tape, no rewind on close.
	* nht1 major 37, minor 129 second IDE tape, no rewind on close.
	* ...
	*
	* The general magnetic tape commands compatible interface, as defined by
	* include/linux/mtio.h, is accessible through the character device.
	*
	* General ide driver configuration options, such as the interrupt-unmask
	* flag, can be configured by issuing an ioctl to the block device interface,
	* as any other ide device.
	*
	* Our own ide-tape ioctl's can be issued to either the block device or
	* the character device interface.
	*
	* Maximal throughput with minimal bus load will usually be achieved in the
	* following scenario:
	*
	* 1. ide-tape is operating in the pipelined operation mode.
	* 2. No buffering is performed by the user backup program.
	*
	* Testing was done with a 2 GB CONNER CTMA 4000 IDE ATAPI Streaming Tape Drive.
	*
	* Here are some words from the first releases of hd.c, which are quoted
	* in ide.c and apply here as well:
	*
	* \| Special care is recommended. Have Fun!
	*
	*
	* An overview of the pipelined operation mode.
	*
	* In the pipelined write mode, we will usually just add requests to our
	* pipeline and return immediately, before we even start to service them. The
	* user program will then have enough time to prepare the next request while
	* we are still busy servicing previous requests. In the pipelined read mode,
	* the situation is similar - we add read-ahead requests into the pipeline,
	* before the user even requested them.
	*
	* The pipeline can be viewed as a "safety net" which will be activated when
	* the system load is high and prevents the user backup program from keeping up
	* with the current tape speed. At this point, the pipeline will get
	* shorter and shorter but the tape will still be streaming at the same speed.
	* Assuming we have enough pipeline stages, the system load will hopefully
	* decrease before the pipeline is completely empty, and the backup program
	* will be able to "catch up" and refill the pipeline again.
	*
	* When using the pipelined mode, it would be best to disable any type of
	* buffering done by the user program, as ide-tape already provides all the
	* benefits in the kernel, where it can be done in a more efficient way.
	* As we will usually not block the user program on a request, the most
	* efficient user code will then be a simple read-write-read-... cycle.
	* Any additional logic will usually just slow down the backup process.
	*
	* Using the pipelined mode, I get a constant over 400 KBps throughput,
	* which seems to be the maximum throughput supported by my tape.
	*
	* However, there are some downfalls:
	*
	* 1. We use memory (for data buffers) in proportional to the number
	* of pipeline stages (each stage is about 26 KB with my tape).
	* 2. In the pipelined write mode, we cheat and postpone error codes
	* to the user task. In read mode, the actual tape position
	* will be a bit further than the last requested block.
	*
	* Concerning (1):
	*
	* 1. We allocate stages dynamically only when we need them. When
	* we don't need them, we don't consume additional memory. In
	* case we can't allocate stages, we just manage without them
	* (at the expense of decreased throughput) so when Linux is
	* tight in memory, we will not pose additional difficulties.
	*
	* 2. The maximum number of stages (which is, in fact, the maximum
	* amount of memory) which we allocate is limited by the compile
	* time parameter IDETAPE_MAX_PIPELINE_STAGES.
	*
	* 3. The maximum number of stages is a controlled parameter - We
	* don't start from the user defined maximum number of stages
	* but from the lower IDETAPE_MIN_PIPELINE_STAGES (again, we
	* will not even allocate this amount of stages if the user
	* program can't handle the speed). We then implement a feedback
	* loop which checks if the pipeline is empty, and if it is, we
	* increase the maximum number of stages as necessary until we
	* reach the optimum value which just manages to keep the tape
	* busy with minimum allocated memory or until we reach
	* IDETAPE_MAX_PIPELINE_STAGES.
	*
	* Concerning (2):
	*
	* In pipelined write mode, ide-tape can not return accurate error codes
	* to the user program since we usually just add the request to the
	* pipeline without waiting for it to be serviced. In case an error
	* occurs, I will report it on the next user request.
	*
	* In the pipelined read mode, subsequent read requests or forward
	* filemark spacing will perform correctly, as we preserve all blocks
	* and filemarks which we encountered during our excess read-ahead.
	*
	* For accurate tape positioning and error reporting, disabling
	* pipelined mode might be the best option.
	*
	* You can enable/disable/tune the pipelined operation mode by adjusting
	* the compile time parameters below.
	*
	*
	* Possible improvements.
	*
	* 1. Support for the ATAPI overlap protocol.
	*
	* In order to maximize bus throughput, we currently use the DSC
	* overlap method which enables ide.c to service requests from the
	* other device while the tape is busy executing a command. The
	* DSC overlap method involves polling the tape's status register
	* for the DSC bit, and servicing the other device while the tape
	* isn't ready.
	*
	* In the current QIC development standard (December 1995),
	* it is recommended that new tape drives will in addition
	* implement the ATAPI overlap protocol, which is used for the
	* same purpose - efficient use of the IDE bus, but is interrupt
	* driven and thus has much less CPU overhead.
	*
	* ATAPI overlap is likely to be supported in most new ATAPI
	* devices, including new ATAPI cdroms, and thus provides us
	* a method by which we can achieve higher throughput when
	* sharing a (fast) ATA-2 disk with any (slow) new ATAPI device.
	*/