Ang: Re: [Stgt-devel] Re: [Iscsitarget-devel] stgt a new version of iscsi target?

Ming Zhang mingz at
Fri Dec 9 16:00:11 CET 2005

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.

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