[Stgt-devel] User-mode iSER

Ming Zhang mingz
Wed Aug 2 16:19:43 CEST 2006

On Wed, 2006-08-02 at 09:01 -0500, Tom Tucker wrote:
> [...snip...]
> > > 
> > > I think this is the area where we will need to get fancy if we want
> > > higher performance. To avoid the copy, we would have to migrate to
> > > netchannels (if they every happen) or implement our own simple tear-away
> > > buffer scheme on top of a socket. I think this is phase-ii, however. 
> > 
> > ok, otherwise copy to user space and copy back to kernel for disk = low
> > performance. yes, direct io can be used here, but then u lose whole
> > cache benefits.
> Can you elaborate on the loss of "whole cache benefits". 

if you use a Linux box as a storage server, it will be desired to use
page cache as storage cache. though this will bring data integrity
issues, but so many people still want to have it for specific

> > you need an in-kernel target mode driver for FCP HBA. and i t shink this
> > are one type of transports.
> One possibility is to move up the kernel/user line to just above the
> network provider layer and make this a zero-copy interface. This would
> support zcopy for TCP, FC and RDMA. 
> I don't think you would want this to be a netlink based interface,
> however, I think you would want it to be a syscall (or ioctl)
> interface...and this makes me worry that the Linux kernel guys wouldn't
> like that.

i could be wrong, in previous stgt design, the data need not to be
copied to user space. but there are needs that process/transforming data
before writing it to disk. for that, you need a zero copy for sure. as
you said, kernel guy maybe dislike both.

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