On 03/19/2010 10:57 PM, Michael Reed wrote: > > > Vladislav Bolkhovitin wrote: >> Douglas Gilbert, on 03/18/2010 10:30 PM wrote: >>>>> Hi Boaz, >>>>> >>>>> > I have fixed that for bsg long ago. There is a flag >>>>> > that you put: >>>>> > sg.flags = BSG_FLAG_Q_AT_TAIL; >>>>> > Which will do the by order, queue at tail. Zero was kept compatible >>>>> > to sg meaning "queue at head" i.e. jump the line. >>>>> > >>>>> > It could be added to sg as well if needed >>>>> >>>>> I think that would be a huge improvement for disk drives. >>>>> >>>>> Isn't BSG driven with a blocking ioctl? Maybe I'm using old docs, but >>>>> from the HOWTO's I've read, I was under the impression it would issue >>>>> one SCSI command at a time and block until it completed. Queueing >>>>> would effectively only be possible then with multiple threads, right? >>>>> >>>> There is a way to do write with multiple ios at once and then >>>> do reads to get return status. (I have untested code that does >>>> that but never needed it so badly as to finish it. But I know >>>> of other people that did it) >>>> >>>> bsg does not have all these mmap/shared buffers options like >>>> sg does, though. Hell not even AIOs. >>>> >>>>> Since sg allows multiple (only 16 right now) ios to be queued >>>>> nonblocking to a block device from a single threaded process, >>>>> an SG_FLAG_Q_AT_TAIL flag would seem particularly relevant. >>>>> >>>> Yes. Currently the queue of 16 does nothing because in effect the >>>> Kernel will completely drain the Q on each "Q_AT_HEAD". So the Q >>>> does nothing for IO with current sg. >>>> >>>>> Nonblocking behavior is the primary reason why I am using sg... it >>>>> seems really inefficient and complex to use dozens of threads and ipc >>>>> just to try to submit a queue of ios to the kernel and hence a device. >>>>> >>>> There are surely other ways to do AIO, no? >>>> (The A in AIO stands for Asynchronous) >>>> >>>>> If bsg actually allowed tasks to be queued nonblocking like sg, I'd >>>>> switch in a heart beat since having two filesystem names for the same >>>>> device introduces locking problems, name translation complexity, and >>>>> confusion in general for administrators. >>>>> >>>> Well bsg has it's own naming translation. You do not use /dev/sdX1. You >>>> need to open the bsg char-device corresponding the particular scsi-device >>> What might be useful is to tell the sd driver not to hook >>> a particular logical unit with peripheral device type 0 >>> (i.e. a disk). Perhaps this could be done via the LU >>> UUID. That way udev (?) would be less tempted to send >>> crap commands to the device at random times. >>> >>> After that you only have to worry about the device node >>> duality between bsg and sg :-) >>> They are pretty well behaved, only sending commands when >>> they are asked to. >>> >>> >>> Adding a SG_FLAG_Q_AT_TAIL flag to sg seems like a >>> good idea. >> >> Great so see the problem confirmed and it's going to move! I and other >> people have been asking this for 3 (or 4?) years and have had a silence >> in reply. It hasn't even been confirmed, so I'd prepare a patch. >> >> Eventually, this should be fixed. >> >> Vlad > > > We're looking at supporting a non-disk scsi device which can effectively > process a queue depth of nine in parallel. Having our application access > the hardware via the "sg" device is our first choice on how to proceed. > > My reading of this thread indicates that the 9 parallel commands we'll > issue will be effectively single threaded by the kernel due to their > being queued at the head of the queue. > > I'd like to encourage the implementation of SG_FLAG_Q_AT_TAIL for > sg devices. > Why not use bsg, then? > Mike Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
