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Blktap Userspace Tools + Library

Andrew Warfield and Julian Chesterfield
16th June 2006


The blktap userspace toolkit provides a user-level disk I/O
interface. The blktap mechanism involves a kernel driver that acts
similarly to the existing Xen/Linux blkback driver, and a set of
associated user-level libraries.  Using these tools, blktap allows
virtual block devices presented to VMs to be implemented in userspace
and to be backed by raw partitions, files, network, etc.

The key benefit of blktap is that it makes it easy and fast to write
arbitrary block backends, and that these user-level backends actually
perform very well.  Specifically:

- Metadata disk formats such as Copy-on-Write, encrypted disks, sparse
  formats and other compression features can be easily implemented.

- Accessing file-based images from userspace avoids problems related
  to flushing dirty pages which are present in the Linux loopback
  driver.  (Specifically, doing a large number of writes to an
  NFS-backed image don't result in the OOM killer going berserk.)

- Per-disk handler processes enable easier userspace policing of block
  resources, and process-granularity QoS techniques (disk scheduling
  and related tools) may be trivially applied to block devices.

- It's very easy to take advantage of userspace facilities such as
  networking libraries, compression utilities, peer-to-peer
  file-sharing systems and so on to build more complex block backends.

- Crashes are contained -- incremental development/debugging is very

How it works (in one paragraph):

Working in conjunction with the kernel blktap driver, all disk I/O
requests from VMs are passed to the userspace deamon (using a shared
memory interface) through a character device. Each active disk is
mapped to an individual device node, allowing per-disk processes to
implement individual block devices where desired.  The userspace
drivers are implemented using asynchronous (Linux libaio),
O_DIRECT-based calls to preserve the unbuffered, batched and
asynchronous request dispatch achieved with the existing blkback
code.  We provide a simple, asynchronous virtual disk interface that
makes it quite easy to add new disk implementations.

As of June 2006 the current supported disk formats are:

 - Raw Images (both on partitions and in image files)
 - File-backed Qcow disks
 - Standalone sparse Qcow disks
 - Fast shareable RAM disk between VMs (requires some form of cluster-based 
   filesystem support e.g. OCFS2 in the guest kernel)
 - Some VMDK images - your mileage may vary

Raw and QCow images have asynchronous backends and so should perform
fairly well.  VMDK is based directly on the qemu vmdk driver, which is
synchronous (a.k.a. slow).

Build and Installation Instructions

Make to configure the blktap backend driver in your dom0 kernel.  It
will cooperate fine with the existing backend driver, so you can
experiment with tap disks without breaking existing VM configs.

To build the tools separately, "make && make install" in 

Using the Tools

Prepare the image for booting. For qcow files use the qcow utilities
installed earlier. e.g. qcow-create generates a blank standalone image
or a file-backed CoW image. img2qcow takes an existing image or
partition and creates a sparse, standalone qcow-based file.

The userspace disk agent is configured to start automatically via xend
(alternatively you can start it manually => 'blktapctrl')

Customise the VM config file to use the 'tap' handler, followed by the
driver type. e.g. for a raw image such as a file or partition:

disk = ['tap:aio:<FILENAME>,sda1,w']

e.g. for a qcow image:

disk = ['tap:qcow:<FILENAME>,sda1,w']

Mounting images in Dom0 using the blktap driver
Tap (and blkback) disks are also mountable in Dom0 without requiring an
active VM to attach. You will need to build a xenlinux Dom0 kernel that
includes the blkfront driver (e.g. the default 'make world' or 
'make kernels' build. Simply use the xm command-line tool to activate
the backend disks, and blkfront will generate a virtual block device that
can be accessed in the same way as a loop device or partition:

e.g. for a raw image file <FILENAME> that would normally be mounted using
the loopback driver (such as 'mount -o loop <FILENAME> /mnt/disk'), do the

xm block-attach 0 tap:aio:<FILENAME> /dev/xvda1 w 0
mount /dev/xvda1 /mnt/disk        <--- don't use loop driver

In this way, you can use any of the userspace device-type drivers built
with the blktap userspace toolkit to open and mount disks such as qcow
or vmdk images:

xm block-attach 0 tap:qcow:<FILENAME> /dev/xvda1 w 0
mount /dev/xvda1 /mnt/disk

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