On 09/07/2010 04:41 PM, Anthony Liguori wrote:
Hi,
We've got copy-on-read and image streaming working in QED and before going much further, I wanted to bounce some interfaces off of the libvirt folks to make sure our final interface makes sense.
Here's the basic idea:
Today, you can create images based on base images that are copy on write. With QED, we also support copy on read which forces a copy from the backing image on read requests and write requests.
Is copy on read QED specific? It looks very similar to the commit command, except with I/O directions reversed. IIRC, commit looks like for each sector: if image.mapped(sector): backing_image.write(sector, image.read(sector)) whereas copy-on-read looks like: def copy_on_read(): set_ioprio(idle) for each sector: if not image.mapped(sector): image.write(sector, backing_image.read(sector)) run_in_thread(copy_on_read) With appropriate locking.
In additional to copy on read, we introduce a notion of streaming a block device which means that we search for an unallocated region of the leaf image and force a copy-on-read operation.
The combination of copy-on-read and streaming means that you can start a guest based on slow storage (like over the network) and bring in blocks on demand while also having a deterministic mechanism to complete the transfer.
The interface for copy-on-read is just an option within qemu-img create. Streaming, on the other hand, requires a bit more thought. Today, I have a monitor command that does the following:
stream <device> <sector offset>
Which will try to stream the minimal amount of data for a single I/O operation and then return how many sectors were successfully streamed.
The idea about how to drive this interface is a loop like:
offset = 0; while offset < image_size: wait_for_idle_time() count = stream(device, offset) offset += count
This is way too low level for the management stack. Have you considered using the idle class I/O priority to implement this? That would allow host-wide prioritization. Not sure how to do cluster-wide, I don't think NFS has the concept of I/O priority. -- error compiling committee.c: too many arguments to function