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authorMichael Roth <mdroth@linux.vnet.ibm.com>2012-04-18 16:28:01 -0500
committerMichael Roth <mdroth@linux.vnet.ibm.com>2012-04-30 08:42:22 -0500
commitf789aa7baff33e74c549a249aba3ae7a364d7642 (patch)
treef46985e57f1a44dc083d001136c2653c39e7f0d2 /cursor_hidden.xpm
parentf22d85e9e67262db34504f4079745f9843da6a92 (diff)
qemu-ga: persist tracking of fsfreeze state via filesystem
Currently, qemu-ga may die/get killed/go away for whatever reason after guest-fsfreeze-freeze has been issued, and before guest-fsfreeze-thaw has been issued. This means the only way to unfreeze the guest is via VNC/network/console access, but obtaining that access after-the-fact can often be very difficult when filesystems are frozen. Logins will almost always hang, for instance. In many cases the only recourse would be to reboot the guest without any quiescing of volatile state, which makes this a corner-case worth giving some attention to. A likely failsafe for this situation would be to use a watchdog to restart qemu-ga if it goes away. There are some precautions qemu-ga needs to take in order to avoid immediately hanging itself on I/O, however, namely, we must disable logging and defer to processing/creation of user-specific logfiles, along with creation of the pid file if we're running as a daemon. We also need to disable non-fsfreeze-safe commands, as we normally would when processing the guest-fsfreeze-freeze command. To track when we need to do this in a way that persists between multiple invocations of qemu-ga, we create a file on the guest filesystem before issuing the fsfreeze, and delete it when doing the thaw. On qemu-ga startup, we check for the existance of this file to determine the need to take the above precautions. We're forced to do it this way since a more traditional approach such as reading/writing state to a dedicated state file will cause access/modification time updates, respectively, both of which will hang if the file resides on a frozen filesystem. Both can occur even if relatime is enabled. Checking for file existence will not update the access time, however, so it's a safe way to check for fsfreeze state. An actual watchdog-based restart of qemu-ga can itself cause an access time update that would thus hang the invocation of qemu-ga, but the logic to workaround that can be handled via the watchdog, so we don't address that here (for relatime we'd periodically touch the qemu-ga binary if the file $qga_statedir/qga.state.isfrozen is not present, this avoids qemu-ga updates or the 1 day relatime threshold causing an access-time update if we try to respawn qemu-ga shortly after it goes away) Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
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