On Thu, 2005-12-08 at 21:57 -0600, Mike Christie wrote: > James Bottomley wrote: > > > > Additionally, it's perfectly possible for all of this to be done zero > > copy on the data. A user space target mmaps the data on its storage > > device and then does a SG_IO type scatter gather user virtual region > > pass to the underlying target infrastructure. We already have this > > demonstrated in the SG_IO path, someone just needs to come up with the > > correct implementation for a target path. > > > > I guess I am going to try to do some work in userspace so we can > benchmark things and see how much performance drops. > > For your suggestion, are you referring to the sg.c mmap path? I was > thinking that maybe we could modify dm so it can do mmap like sg.c does. > The dm device would be the target device and would sit above the real > device or another MD/DM/Loop/ramdsik device or whatever so when > userspace decides to execute the command it would inform the dm device > and that kernel driver would just then send down the bios. > > For something like qlogic or mpt would this basically work like the > following: > > 1. dm mmap is called and our dm_target (dm drivers like dm-multipath or > dm-raid are called dm_targets) does like sg.c sg_mmap* and sg_vma* ops. > 2. HW interrupt comes in and we allocate a scatterlist with pages from #1. > 3. netlink (or whatever is the favorite interface) a message to > userpsace to tell it we have a command ready. > 4. userspace decides if it is a read or write and if it is a read or > write then userspace tells the dm device to read/write some pages. > 5. dm's bi_endio is called when io is finished so we netlink to > userspace and then userspaces netlinks back to the kernel and tells the > LLD like qlogic that some data and/or a responce or sense is ready for > it to transfer. we can count how many system calls and context switches will occur for each request here. will this be a source of high response time? i guess we really need to have both a kernel and user space implementation here to compare. - : 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
