Thanks for pointing me to the comments, Shriram. That was very helpful.
What i am unable to figure out is how does Remus tell the module to only release packets belonging to a given epoch ? I am looking at the BufferedNIC class in xen-unstable/tools/python/xen/remus/device.py and the postsuspend method calls self._sendqmsg(qdisc.TC_PLUG_CHECKPOINT) while the commit method calls self._sendqmsg(qdisc.TC_PLUG_RELEASE). Where is it telling that only packets of the last epoch should be released ?
The release command tells the module that the packets of last epoch should be released. (implicit). It is called after the checkpoint command for the next epoch has been issued in the postsuspend phase.There is an ASCII diagram on the modules header that shows the sequence. Also, as you said that module starts out with buffering everything. So if i comment out the self._sendqmsg(qdisc.TC_PLUG_CHECKPOINT) but keep the self._sendqmsg(qdisc.TC_PLUG_RELEASE), there should be no network buffering at all, but i still see that all packets are buffered.
To understand this, you need to read the sch_plug.c source
Thanking you, Rahul. On Apr 19, 2012, at 5:24 PM, Shriram Rajagopalan wrote: On Thu, Apr 19, 2012 at 9:15 AM, Rahul Singh <singh.rahul.1983@xxxxxxxxx> wrote:
Hi,
I am trying to understand and change the network buffering that is being
used by Remus, the HA solution present in Xen. From what i understood from reading the code, Remus
calls the postsuspend method of the BufferedNIC after it suspends the
domain that sends TC_PLUG_CHECKPOINT message and start the buffering for packets in the "next" epoch [the one that starts after the domain is resumed]
and
then calls the commit method of BufferedNIC after it gets the
acknowledgement that sends the TC_PLUG_RELEASE message and releases the
buffered packets. of the previous current epoch [the one whose checkpoint was just committed on the backup machine]
I have the following doubts
1) How does remus ensure that the packets gets buffered for an entire epoch ?
2) If i comment out the lines in the "postsuspend" and "commit" lines of
the BufferedNIC class that send the TC_PLUG_CHECKPOINT and
TC_PLUG_RELEASE, i see that all the network packets are being buffered
and i cannot ping the Remus-protected VM at all. Where is the packet
buffering happen if i comment out these 2 lines ?
the module starts out in the buffered mode and releases packets only upon receiving a RELEASE command.
hope that helps.
cheers
shriram
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