diff --git a/real_dirty_memory_accounter.hh b/real_dirty_memory_accounter.hh
index 461ce10b39..6bd74fb65c 100644
--- a/real_dirty_memory_accounter.hh
+++ b/real_dirty_memory_accounter.hh
@@ -12,7 +12,81 @@
 #include "row_cache.hh"
 #include "replica/dirty_memory_manager.hh"
 
-// makes sure that cache update handles real dirty memory correctly.
+// # Flush control overview
+//
+// It is good for memtables to grow as big as possible before they are flushed
+// to disk, because this reduces the number of sstables, which in turn reduces
+// the number of disk reads on queries and/or reduces the necessary compaction
+// work.
+//
+// On the other hand, the flush should start early enough to leave enough space
+// in RAM for new data incoming during the flush. Otherwise RAM could be
+// overfilled and the database would have to be throttled until some space is
+// freed.
+//
+// It is good for a memtable flush to happen as slow as possible, because it's
+// a non-interactive task, which should take as little resources from
+// interactive tasks as possible.
+//
+// On the other hand, the flush has to progress fast enough to keep up with
+// incoming writes, otherwise the size of data in RAM will keep growing
+// until RAM is overfilled.
+//
+// The balance of the above is kept by dirty_memory_manager and flush_controller.
+// They attempt to make flushes as delayed and slow as possible without risking
+// an OOM situation.
+//
+// # Flush control implementation
+//
+// Flush delay and speed is based on some formulas involving total available RAM,
+// total ("real") memtable memory and "unspooled" (not flushed to disk yet) memtable
+// memory. See dirty_memory_manager for details.
+//
+// ("Dirty" is a term borrowed from general caching terminology, which usually means
+// cache entries which were modified and have to be written back before being discarded.
+// In Scylla, "dirty memory" simply means "memory taken up by memtable data").
+//
+// (In some context the implementation might find it more natural to talk about
+// "spooled" memory instead of "unspooled". "Spooled" is just the difference between
+// "real" and "unspooled").
+//
+// Every memtable is kept in its own LSA region which tracks exact (the allocator is
+// the source of truth about memory usage) "real" changes for that memtable.
+// While the memtable is active, its "unspooled" is (obviously) equal to its "real".
+//
+// Once flush starts, remaining "unspooled" is tracked by flush_memory_accounter.
+// As flush_reader reads a memtable and passes its data to an sstable writer,
+// it asks flush_memory_accounter to decrement "unspooled" memory counters.
+//
+// The accounting by flush_memory_accounter is not exact (for example it
+// doesn't account the memtable tree nodes, only the contents stored there) but
+// it doesn't have to. It is only used by flush control formulas, so it only
+// has to be accurate enough for the heurisitcs to work.
+//
+// When memtable flush is finished, the amount of "unspooled" memory is corrected
+// from its inexact value to now-exact 0.
+//
+// After a memtable flush finishes, the memtable has to be merged into the
+// cache, to update or invalidate existing cache entries, so combined RAM data
+// stays up to date with sstables after the memtable disappears.
+//
+// During this merge, "real" decreases, and this has to be accounted for.
+// Unfortunately we can't rely on LSA's counters (which are the source of
+// truth for memory accounting) for "real", because the merge
+// requires the memtable LSA region to be merged into the cache LSA region.
+// As soon as the merge starts, all memtable data is considered a part of the cache
+// by the LSA.
+//
+// So, similarly as with flush_memory_accounter earlier, we have real_dirty_memory_accounter
+// whose job is to incrementally decrease "real" as data is merged into the cache.
+// This might also be slightly inexact.
+//
+// As row_cache::update() progresses, it estimates the amount of processed memory
+// as well as it can, and asks real_dirty_memory_accounter to "unpin" it from "real".
+//
+// Once the merge finishes, the "real" for the merged memtable is reduced from
+// its slightly inexact value to now-exact 0.
+//
 class real_dirty_memory_accounter {
     replica::dirty_memory_manager& _mgr;
     cache_tracker& _tracker;