You can set the value of TTL for set-level f4m to a high value (a few hours). However, ensure that the TTL for the stream-level f4m is low, preferably half the duration of the fragment. For a stream with fragment duration four seconds, stream level f4m should have a TTL of two seconds. You should set the TTL for set-level f4m to a low value (for example, half the fragment duration) when the client-load must be divided equally or in a pre-defined ratio between the data centers. You configure this ratio at GTM.
If the set-level f4m is served from the static folder at the origin server, you can set TTL in the origin.xml. When set level f4m resides in a container level, you can set TTL in container.xml.
Any TTL that you set for f4m in the origin.xml file applies to all the f4m requests served from any of the modules of the origin server. To achieve a lower TTL for stream-level f4m, configure the TTL for the stream-level f4m in the stream module's configuration. Alternatively, modify the varnish script to configure set-level f4m TTL.