[#1745] writecache: Actualize docs

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

pkg/local_object_storage/writecache/doc.go

@@ -1,20 +1,11 @@
 // 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).
+// Write-cache has 2 components:
+// 1. Key-value (bbolt) database for storing small objects.
+// 2. Filesystem tree for storing big objects.
 //
-// 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.
+// Flushing from the writecache to the main storage is done in the background.
+// To make it possible to serve Read requests after the object was flushed,
+// we maintain an LRU cache containing addresses of all the objects that
+// could be safely deleted. The actual deletion is done during eviction from this cache.
 package writecache

pkg/local_object_storage/writecache/flush.go

@@ -185,9 +185,7 @@ func (c *cache) flushBigObjects() {
 	}
 }
 
-// flushWorker runs in a separate goroutine and write objects to the main storage.
-// If flushFirst is true, flushing objects from cache database takes priority over
-// putting new objects.
+// flushWorker writes objects to the main storage.
 func (c *cache) flushWorker(_ int) {
 	defer c.wg.Done()