Hot memory buffer has two major functions. First, it caches incoming (new) documents. Second it caches documents swapped in from disk as a result of a disk hit. It is not obvious what documents should be cached in memory if any. To further understand how Squid memory buffer works, we track down the memory hits that correspond to documents that were never swapped in:

When a document is retrieved from its source, it is placed into memory buffer. Later, a document is swapped out to free space for incoming requests. However, before the document content is freed from memory buffer, it may be requested by other clients. Such requests result in no swap-in memory hits that we are interested in. These hits are interesting because they show how effective memory buffer is in caching new (previously uncached) documents.