Each time the auxiliary index becomes too large, we merge it into the main
index. The cost of this merging operation depends on how we store the index
in the file system. If we store each postings list as a separate file, then the
merge simply consists of extending each postings list of the main index by
the corresponding postings list of the auxiliary index. In this scheme, the
reason for keeping the auxiliary index is to reduce the number of disk seeks
required over time. Updating each document separately requires up to Mave
disk seeks, where Mave is the average size of the vocabulary of documents in
the collection. With an auxiliary index, we only put additional load on the disk when we merge auxiliary and main indexes.