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blktap, vhd stuff
This project forked from jonludlam/blktap
blktap, vhd stuff
Blktap Userspace Tools + Library ================================ Andrew Warfield and Julian Chesterfield 16th June 2006 {firstname.lastname}@cl.cam.ac.uk 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 fast. 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 tools/blktap. 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 following: 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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