On 4/30/13 3:26 PM, "Dennis Lawler" <dlaw...@gmail.com> wrote:
>Every time a new VM is started up, there is a 2 second outage in DNS >services that can cause problems in guest VMs that use the router VM for >DNS. > > > >For Cloudstack configurations using both DHCP and DNS services on the >router >VM (both implemented with dnsmasq), there is currently a 2 second DNS >service outage every time a new VM is instantiated > > > >The source of this outage is in edithosts.sh, which uses "service dnsmasq >restart" to pick up the freshly added DNS and DHCP entries. > >Restarting the dnsmasq service triggers a sleep for 2 seconds after >killing >dnsmasq before starting it back up again. > > > >An obvious solution would be to replace "service dnsmasq restart" with >"kill >-s 1 $pid" (SIGHUP) so that dnsmasq reads the new DHCP entries without >restarting, as in dnsmasq_edithosts.sh (external dhcp). > > >Unfortunately, this solution is flawed because dnsmasq SIGHUP handling >does >not expire in-memory DHCP leases in dnsmasq and all leases are infinite by >default. Aha! That's why SIGHUP didn't work consistently. This has been bugging me for a long time. >Thus, this will only work if the guest VM performs a DHCP release on >shutdown, which cannot always be guaranteed. > > > >A few possible solutions off the top of my head: > >1. Separate DNS and DHCP services. While DHCP services still >experience an outage during VM, DNS will not necessarily be impacted if >implemented correctly. > >2. Use SIGHUP with dnsmasq and implement a removeDhcpEntry interface >for network appliances to force a DHCP release whenever a NIC / IP is >deallocated. This can use dhcp_release to simulate a DHCP release on the >router VM. >Catch: dhcp_release is not available for Debian 6.0. The System VM needs >to >be updated to at least Debian 7.0, or the dnsmasq-tools .deb from 7.0 >would >need to be included in the System VM image. There is going to be a new system vm based on 7.0 for the upcoming release. This should work with earlier releases as well. https://cwiki.apache.org/confluence/x/UlHVAQ > >3. Change DHCP to have a shorter lease, track de-allocation of IPs >separately from VM destruction. >Catch: This may cause occasional IP pool exhaustion depending on >allocation >of the guest IP range and the rate of VM destruction / instantiation in >the >network. > > > >Thoughts? >
