[#472] blobstor: implement write-cache

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/writecache/doc.go

@@ -0,0 +1,20 @@
+// Package writecache implements write-cache for objects.
+//
+// It contains in-memory cache of fixed size and underlying database
+// (usually on SSD) for storing small objects.
+// There are 3 places where object can be:
+// 1. In-memory cache.
+// 2. On-disk cache DB.
+// 3. Main storage (blobstor).
+//
+// There are 2 types of background jobs:
+// 1. Persisting objects from in-memory cache to database.
+// 2. Flushing objects from database to blobstor.
+//	  On flushing object address is put in in-memory LRU cache.
+//	  The actual deletion from the DB is done when object
+//	  is evicted from this cache.
+//
+// Putting objects to the main storage is done by multiple workers.
+// Some of them prioritize flushing items, others prioritize putting new objects.
+// The current ration is 50/50. This helps to make some progress even under load.
+package writecache