Xen devel - May 2009 - RE: [Xen-changelog] [xen-unstable] blktap2: a completely rewritten blktap implementation

Just wondering... has this been tested with multiple domains
sharing the same blktap device?  (e.g. specifying
tap:aio:/pathto/file.img,hda,w! from two different domains
in a cluster)
> -----Original Message-----
> From: Xen patchbot-unstable
> [mailto:patchbot-unstable@lists.xensource.com]
> Sent: Wednesday, May 27, 2009 5:31 AM
> To: xen-changelog@lists.xensource.com
> Subject: [Xen-changelog] [xen-unstable] blktap2: a completely 
> rewritten
> blktap implementation
> 
> 
> # HG changeset patch
> # User Keir Fraser <keir.fraser@citrix.com>
> # Date 1243335151 -3600
> # Node ID 1c627434605e7747689047e1761c193ceb4f9ef0
> # Parent  f210a633571c17c1a5e47980d53b00b0cab5b726
> blktap2: a completely rewritten blktap implementation
> 
> Benefits to blktap2 over the old version of blktap:
> 
> * Isolation from xenstore - Blktap devices are now created directly on
>    the linux dom0 command line, rather than being spawned in response
>    to XenStore events.  This is handy for debugging, makes blktap
>    generally easier to work with, and is a step toward a generic
>    user-level block device implementation that is not Xen-specific.
> 
> * Improved tapdisk infrastructure: simpler request forwarding, new
>    request scheduler, request merging, more efficient use of AIO.
> 
> * Improved tapdisk error handling and memory management.  No
>    allocations on the block data path, IO retry logic to protect
>    guests
>    transient block device failures.  This has been tested and is known
>    to work on weird environments such as NFS soft mounts.
> 
> * Pause and snapshot of live virtual disks (see xmsnap script).
> 
> * VHD support.  The VHD code in this release has been rigorously
>    tested, and represents a very mature implementation of the VHD
>    image
>    format.
> 
> * No more duplication of mechanism with blkback.  The blktap kernel
>    module has changed dramatically from the original blktap.  Blkback
>    is now always used to talk to Xen guests, blktap just presents a
>    Linux gendisk that blkback can export.  This is done while
>    preserving the zero-copy data path from domU to physical device.
> 
> These patches deprecate the old blktap code, which can hopefully be
> removed from the tree completely at some point in the future.
> 
> Signed-off-by: Jake Wires <jake.wires@citrix.com>
> Signed-off-by: Dutch Meyer <dmeyer@cs.ubc.ca>
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Dan Magenheimer wrote:
> Just wondering... has this been tested with multiple domains
> sharing the same blktap device?  (e.g. specifying
> tap:aio:/pathto/file.img,hda,w! from two different domains
> in a cluster)
This feature is really important at least for stubdoms and windows pv
drivers.
However in this particular case the stubdom and the pv drivers in the
guest will never open the disk at the same time because when the pv
drivers start they ask qemu to close the disk.

So far I have been doing some testing with stubdoms, blktap2 and vhd and
they seem to work well together.

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