Re: AFS-based VBD backend

[Andrew Warfield <andrew.warfield@gmail.com>]

> The blocktap backend driver is what you want. I'm not sure of it's
> current state, but the plan is to enable you to terminate a blk device
> channel in user-space.

The driver currently does this.  There is a rather dated version of it
in the unstable tree...  It lets you either terminate a blkif
connection in userspace of a tap domain, or intercept in-flight
requests by proxying them to user-space before they are passed along
to a normal backend.  Data pages provided by the front end are mapped
up to user space by the tap driver, so there is no copying along that
path.

I've got a newer and more, um, robust ;) version of the blktap stuff
in my repository... it's a bit tied up with some of the other device
channel updates that we have been looking at lately though -- so it's
not in a state to integrate directly to the public trees.  I'd imagine
that much of this stuff should be ready to go into unstable in the
next few weeks...  hopefully along with a nettap as well.

Steve, if you need the tap more urgently than this, I can likely sort
out an interim version.  If you can wait until early january though,
that might be best.

> A future revision of blocktap could use kiovec's to avoid having to copy
> data into user space, and thus would give good performance.

arranging direct i/o to the mapped data pages is clearly the thing to
do.  I'll take a look at the kvec stuff over the next few weeks as
well.

a.


-------------------------------------------------------
SF email is sponsored by - The IT Product Guide
Read honest & candid reviews on hundreds of IT Products from real users.
Discover which products truly live up to the hype. Start reading now. 
http://productguide.itmanagersjournal.com/