In this article we discuss how indexes are loaded at startup and the implications on incoming queries and management of the search instance.
In older Sphinx versions, if preopen option was set, indexes would been pre-read and loaded into memory. During this time, the daemon would refuse accepting any incoming connection. If for small indexes this was not a big problem as they would load fast. But for a huge index which would need to load tens or even hundreds of GB, this was a real problem as it could take several minutes or more to finish the loading.
In Sphinx 2.3 this changed, as index files are memory-mapped (with mmap) rather than simply read and loaded into memory. With mmap, a shadow thread of the daemon and not the main one reads first one byte for each page and then continues reading the rest of the data, slowly bringing everything in the RAM but also allowing access to the data. In short, this is a lazy loading of index files.
This way the daemon can accept connections as soon as is up. If a query needs to access some random data that is not loaded yet in memory, it will read it from disk. This also means the query could execute slower than usual (depending on storage performance) but at least the daemon can provide results. Even more, as soon as the files of an index are loaded, the daemon can offer 'full speed' for it even if the other indexes are still loading.
While this is good news for most users, for some the 'lazy' loading might not be best. For example a load balancer using simple strategies (random or round robin) which don't measure response times will add the rebooted instance as soon as the daemon is up. As the queries will require reads directly from disk, the instance will provide higher or a lot higher response time than usual, something that is not desired. Instead it would be better if possible for the instance to be added to the cluster only when daemon finished loading the indexes.
To cover this situation an option for searchd exists since 2.3, but was unfortunately it was undocumented until recently: '--force-preread'. Using this option the daemon will still mmap the index files, but the preread is a blocker operation (just like in old versions) and searchd will not respond until everything is loaded in memory.