[#776] writecache: Limit size of used disk space

There is a need to limit disk space used by write-cache. It is almost
impossible to calculate the value exactly. It is proposed to estimate the
size of the cache by the number of objects stored in it.

Track amounts of objects saved in DB and FSTree separately. To do this,
`ObjectCounters` interface is defined. It is generalized to a store of
numbers that can be made persistent (new option `WithObjectCounters`). By
default DB number is calculated as key number in default bucket, and FS
number is set same to DB since it is currently hard to read the actual value
from `FSTree` instance. Each PUT/DELETE operation to DB or FS
increases/decreases corresponding counter. Before each PUT op an overflow
check is performed with the following formula for evaluating the occupied
space: `NumDB * MaxDBSize + NumFS * MaxFSSize`. If next PUT can cause
write-cache overflow, object is written to the main storage.

By default maximum write-cache size is set to 1GB.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/writecache/options.go

@@ -26,6 +26,11 @@ type options struct {
 	smallObjectSize uint64
 	// workersCount is the number of workers flushing objects in parallel.
 	workersCount int
+	// maxCacheSize is the maximum total size of all objects saved in cache (DB + FS).
+	// 1 GiB by default.
+	maxCacheSize uint64
+	// objCounters is an ObjectCounters instance needed for cache size estimation.
+	objCounters ObjectCounters
 }
 
 // WithLogger sets logger.
@@ -88,3 +93,17 @@ func WithFlushWorkersCount(c int) Option {
 		}
 	}
 }
+
+// WithObjectCounters sets ObjectCounters instance needed for cache write-cache size estimation.
+func WithObjectCounters(v ObjectCounters) Option {
+	return func(o *options) {
+		o.objCounters = v
+	}
+}
+
+// WithMaxCacheSize sets maximum write-cache size in bytes.
+func WithMaxCacheSize(sz uint64) Option {
+	return func(o *options) {
+		o.maxCacheSize = sz
+	}
+}