Re: [libvirt] [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration

On 09/07/2010 09:55 AM, Stefan Hajnoczi wrote: > > On Tue, Sep 7, 2010 at 3:51 PM, Anthony Liguori > <aliguori(a)linux.vnet.ibm.com&gt;  wrote: > >> >> On 09/07/2010 09:33 AM, Stefan Hajnoczi wrote: >> >>> >>> On Tue, Sep 7, 2010 at 2:41 PM, Anthony Liguori >>> <aliguori(a)linux.vnet.ibm.com&gt;    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.