en-US/snapshot-throttling.xml

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<section id="snapshot-throttling">
    <title>Snapshot Job Throttling</title>
    <para>When a snapshot of a virtual machine is requested, the snapshot job runs on the same
        host where the VM is running or, in the case of a stopped VM, the host where it ran last. If
        many snapshots are requested for VMs on a single host, this can lead to problems with too
        many snapshot jobs overwhelming the resources of the host.</para>
    <para>To address this situation, the cloud's root administrator can throttle how many snapshot
        jobs are executed simultaneously on the hosts in the cloud by using the global configuration
        setting concurrent.snapshots.threshold.perhost. By using this setting, the administrator can
        better ensure that snapshot jobs do not time out and hypervisor hosts do not experience
        performance issues due to hosts being overloaded with too many snapshot requests.</para>
    <para>Set concurrent.snapshots.threshold.perhost to a value that represents a best guess about
        how many snapshot jobs the hypervisor hosts can execute at one time, given the current
        resources of the hosts and the number of VMs running on the hosts. If a given host has more
        snapshot requests, the additional requests are placed in a waiting queue. No new snapshot
        jobs will start until the number of currently executing snapshot jobs falls below the
        configured limit.</para>
    <para>The admin can also set job.expire.minutes to place a maximum on how long a snapshot
        request will wait in the queue. If this limit is reached, the snapshot request fails and
        returns an error message. </para>
</section>