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SLA: High Availability timeouts explained
This page provides information on the various arguments used to identify and handle connectivity issues for highly available VMs, both in previous and current oVirt versions.
There are several parameters that control monitoring and reaction behavior for highly available VMs (vdsConnectionTimeout and vdsRetries have been added in 3.2);
VDSAttemptsToResetCount controls how many consecutive VDSM operations have to fail to consider a host a candidate for fencing. Default value is 2.
TimeoutToResetVdsInSeconds controls how long a host has to be failing (VDSM operations failing continuously) to consider it a candidate for fencing. Default values is 60s.
Both above parameters have to be evaluated as true to actually initiate fencing of a host, ie- both exceeded.
vdsTimeout controls how long the engine waits for a VDSM operation to complete, its default value is 180s (3 minutes). This value shouldn’t be changed, as some VDSM operations require that time to complete.
vdsConnectionTimeout controls how long the engine waits for a connection to VDSM to complete. Its default value is 180s (3 minutes). This value should be reduced, and in fact its default value has already been changed to 2s in 3.3. The default value was changed to 20s in 3.6.
vdsRetries controls how many times failed VDSM operations should be retried. Its default value is 3 in 3.2, while in 3.3 it was changed to 0.
With these default values a host that goes completely down and doesn’t respond at all will take at least 6 minutes to be fenced: the reason is that any operation will take those 6 minutes because we retry them 3 times (vdsRetries), each time it takes approx 63s (vdsTimeout, the kernel cuts to 63s any value larger than that), and we need 2 consecutive failures to start fencing: 63sec * 3 * 2 ~ 6min.
For a more predictable fencing the following parameters need to be changed: # engine-config -s vdsConnectionTimeout=2 # engine-config -s vdsRetries=0
With these settings the host should be fenced approx 1 minute after it stops responding, and this happens because now the VDSAttemptsToResetCount and TimeoutToResetVdsInSeconds are really taken into account and not masked by the low level timeouts.