<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"> <head> <meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> <meta name="Generator" content="Microsoft Word 15 (filtered medium)"> <style></style> </head> <body lang="EN-US" link="#0563C1" vlink="#954F72"> <div class="WordSection1"> I am trying to use /etc/libvirt/hooks/qemu to control the startup of several guests with interdependencies. The goal is to delay the start of guest B until the DNS server on guest A is running. To accomplish this, I wrote a qemu hook script that detects the normal startup of guest B and start a second script in the background to wait until the preconditions to start B are fulfilled, then start B using a call to the virsh command.<o:p></o:p> <o:p> </o:p> For this strategy to work, it must handle the case where libvirt has chosen guest B as the first guest to attempt to start. (Although renaming the symlinks in /etc/libvirt/qemu/autostart to force starting the guests in a particular order might work, I do not want to rely on this undocumented behavior). In the case where libvirt happens to attempt to start guest B before it starts guest A, the hook script needs to somehow tell libvirt to skip guest B and go on to starting the next guest. Otherwise a deadlock would result as libvirt waited for B to start, but B was waiting for A to start. I have tried to handle this by returning failure from the hook script for the initial attempt to start B once the background script has been started to implement the DNS check and eventually the delayed start of B.<o:p></o:p> <o:p> </o:p> Unfortunately, I cannot find a way to force libvirt to continue until the background script exits. No combination of background execution, nohup, disown, setsid -f, or at seems to detach the process sufficiently to “fool” libvirt into acting on the “exit 1” line in the qemu script and proceed on to start other guests. As a result, the dependency of B on A deadlocks, and neither guest ever starts.<o:p></o:p> <o:p> </o:p> Can someone please either find an error in my approach or propose a different strategy to implement this customized dependency of the startup of one guest on another?<o:p></o:p> <o:p> </o:p> Thanks –<o:p></o:p> <o:p> </o:p> ======<o:p></o:p> <o:p> </o:p> Here is my qemu script:<o:p></o:p> <o:p> </o:p> #!/bin/bash<o:p></o:p> if [[ "$2" == 'start' ]]; then<o:p></o:p> echo "$0: Starting $1..." |& logger<o:p></o:p> if [[ "$1" == 'B' ]]; then<o:p></o:p> # The next line is where the background script is invoked<o:p></o:p> /bin/bash /usr/local/bin/startB &<o:p></o:p> # These also don’t work:<o:p></o:p> # (/bin/bash /usr/local/bin/startB) & ; disown<o:p></o:p> # setsid -f (/bin/bash /usr/local/bin/startB) & ; disown<o:p></o:p> # Unfortunately, the exit in the following line doesn’t force libvirt to move on to the next guest to start until the background command has itself exited<o:p></o:p> exit 1;<o:p></o:p> fi<o:p></o:p> fi<o:p></o:p> <o:p> </o:p> Here is the startB script, including a call to a program named in the $dnssuccess variable that does the testing of DNS availability on guest A:<o:p></o:p> <o:p> </o:p> #!/bin/bash<o:p></o:p> until $dnssuccess<o:p></o:p> do<o:p></o:p> echo "$0: Delaying start of guest B 10 seconds" |& logger<o:p></o:p> sleep 10;<o:p></o:p> done<o:p></o:p> # It’s now OK to start guest<o:p></o:p> echo "$0: Now starting guest B" |& logger<o:p></o:p> virsh start B;<o:p></o:p> <o:p> </o:p> ===== <o:p></o:p> </div> </body> </html>