The additional section cache, also called acache,
+is an internal cache to improve the response performance of BIND 9.
+When the additional section caching is enabled, BIND 9 will
+cache internal short-cut to the additional section content for each
+answer RR.
+Note that acache is an internal caching mechanism of BIND 9, and is
+not relevant to the DNS caching server function.
+
The additional section caching does not make any difference on the
+response content (except the RRsets ordering of the additional
+section, see below), but can improve the response performance significantly.
+It is particularly effective when BIND 9 acts as an authoritative server
+for a zone that has many delegations with many glue RRs.
+
In order to achieve the maximum performance improvement by acache,
+it is recommended to set additional-from-cache
+to no, since the current implementation of acache
+does not make a short-cut of additional section information from a DNS
+cache data.
+
One obvious disadvantage of acache is that it requires much more
+memory for the internal cached data.
+Thus, if the response performance does not matter and memory
+consumption is much more severe, the acache mechanism can be
+disabled by setting use-additional-cache to
+no.
+It is also possible to specify the upper limit of memory consumption
+for acache by max-acache-size.
+
The additional section caching also has a minor effect on the RRset
+ordering in the additional section.
+Without acache, the "cyclic" order is effective for the additional
+section as well as the answer and authority sections.
+However, the additional section caching fixes the ordering when it
+first caches an RRset for the additional section, and the same
+ordering will be kept in succeeding responses, regardless of the
+configuration for rrset-order.
+This should be minor, though, since an RRset in the additional section
+typically only contains a small number of RRs (and in many cases it
+only contains a single RR), in which case the
+ordering does not matter much.
+
The following is a summary of options related to acache.
+
- use-additional-cache
If yes, the additional section caching is enabled.
+The default value is yes.
+
- acache-cleaning-interval
The server will remove stale cache entries, based on an LRU based
+algorithm, every acache-cleaning-interval minutes.
+The default is 60 minutes.
+If set to 0, no periodic cleaning will occur.
+
- max-acache-size
The maximum amount of memory to use for the server's acache, in bytes.
+When the amount of data in the acache reaches this limit, the server
+will cause more aggressive cleaning so that the limit is not exceeded.
+In a server with multiple views, the limit applies separately to the
+acache of each view.
+The default is unlimited, meaning that
+entries are purged from acache only at the periodic cleaning time.
+