On 09/07/2010 09:55 AM, Stefan Hajnoczi wrote:
On Tue, Sep 7, 2010 at 3:51 PM, Anthony Liguori <aliguori@linux.vnet.ibm.com> wrote:
On 09/07/2010 09:33 AM, Stefan Hajnoczi wrote:
On Tue, Sep 7, 2010 at 2:41 PM, Anthony Liguori <aliguori@linux.vnet.ibm.com> wrote:
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
Obviously, the "wait_for_idle_time()" requires wide system awareness. The thing I'm not sure about is 1) would libvirt want to expose a similar stream interface and let management software determine idle time 2) attempt to detect idle time on it's own and provide a higher level interface. If (2), the question then becomes whether we should try to do this within qemu and provide libvirt a higher level interface.
A self-tuning solution is attractive because it reduces the need for other components (management stack) or the user to get involved. In this case self-tuning should be possible. We need to detect periods of I/O inactivity, for example tracking the number of in-flight requests and then setting a grace timer when it reaches zero. When the grace timer expires, we start streaming until the guest initiates I/O again.
That detects idle I/O within a single QEMU guest, but you might have another guest running that's I/O bound which means that from an overall system throughput perspective, you really don't want to stream.
I think libvirt might be able to do a better job here by looking at overall system I/O usage. But I'm not sure hence this RFC :-)
Isn't this what block I/O controller cgroups is meant to solve? If you give vm-1 50% block bandwidth and vm-2 50% block bandwidth then vm-1 can do streaming without eating into vm-2's guaranteed bandwidth.
That assumes you're capping I/O. But sometimes you care about overall system throughput more than you care about any individual VM. Another way to look at it may be, a user waits for a cron job that runs at midnight and starts streaming as necessary. However, the user wants to be able to interrupt the streaming should there been a sudden demand. If the user drives the streaming through an interface like I've specified, they're in full control. It's pretty simple to build a interfaces on top of this that implement stream as an aggressive or conservative background task too.
Also, I'm not sure we should worry about the priority of the I/O too much: perhaps the user wants their vm to stream more than they want an unimportant local vm that is currently I/O bound to have all resources to itself. So I think it makes sense to defer this and not try for system-wide knowledge inside a QEMU process.
Right, so that argues for an incremental interface like I started with :-) BTW, this whole discussion is also relevant for other background tasks like online defragmentation so keep that use-case in mind too. Regards, Anthony Liguori
Stefan