This patchset contains various measures to improve the speed and usefulness of this driver. It has been used to test the rewrite of the SCSI generic (sg) driver which is still underway. Disk to disk copies are the test of choice by the author. Some testing has been done using real hard disks and SSDs but the bulk of the testing has been done using this driver as both the source and destination of the copy. SSDs have two shortcomings: they are not as fast as the manufacturers would like users to believe with an average latency to READ at around 100 microseconds; the second problem is "endurance". Endurance is a wear-out factor based on the number of WRITEs to the SSD. One would hope both these measures will improve in the future. The author found that precise command duration timing gave a false impression of how "bulletproof" the sg driver state machines and locking was. The first patch involving randomizing the command durations and it did expose various issues in the driver under test (sg). The next issue was the correctness of the bulk copies being done. The doublestore and verify patches allow the copies to be verified and it demonstrated at least one area of concern for the sg driver. Since all scsi_debug memory stores accesses are done in the context of queuecommand() call, the *_irqsave() and *_irqrestore() variants of the associated locks have been removed. That could be a problem if queuecommand() can ever be called form an interrupt or related context. Finally to address the discrepancy between command duration times seen by the sg driver compared to what was set with this driver's ndelay option, this driver's timekeeping for short durations was made more accurate. This patchset is against Martin Petersen's git repository and its 5.6/scsi-queue branch. --- One shortcoming of the doublestore approach is that both stores share all the other module settings that are held at file scope in the driver. With a little extra hacking it is possible to have multiple scsi_debug (like) drivers in a single kernel. That may be one way around this shortcoming. Douglas Gilbert (6): scsi_debug: randomize command completion time scsi_debug: add doublestore option scsi_debug: implement verify(10), add verify(16) scsi_debug: weaken rwlock around ramdisk access scsi_debug: improve command duration calculation scsi_debug: bump to version 1.89 drivers/scsi/scsi_debug.c | 444 +++++++++++++++++++++++++++++--------- 1 file changed, 343 insertions(+), 101 deletions(-) -- 2.24.1
