On Sunday 27 January 2008, Borislav Petkov wrote: > Also, cleanup whitespace and update comments. > > Signed-off-by: Borislav Petkov <bbpetkov@xxxxxxxx> applied with some changes > --- > Documentation/ide/ChangeLog.ide-tape.1995-2002 | 405 +++++++++++++++++++++++ > drivers/ide/ide-tape.c | 414 +----------------------- > 2 files changed, 409 insertions(+), 410 deletions(-) > > diff --git a/Documentation/ide/ChangeLog.ide-tape.1995-2002 b/Documentation/ide/ChangeLog.ide-tape.1995-2002 > new file mode 100644 > index 0000000..e406762 > --- /dev/null > +++ b/Documentation/ide/ChangeLog.ide-tape.1995-2002 > @@ -0,0 +1,405 @@ > +/* > + * IDE ATAPI streaming tape driver. > + * > + * This driver is a part of the Linux ide driver and works in co-operation > + * with drivers/block/ide.c. I removed incorrect reference to drivers/block/ide.c > + * 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. the above is not the part of the changelog [...] > + * 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! [...] ditto > + * 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. > + */ ditto All of the above comments got moved to Documentation/ide/ide-tape.txt. - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
