lib/mergeset: remove a pool for inmemoryBlock structs

The pool for inmemoryBlock struct doesn't give any performance gains in production workloads,
while it may result in excess memory usage for inmemoryBlock structs inside the pool during
background merge of indexdb.

lib/mergeset/encoding.go

@@ -540,17 +540,6 @@ func putLensBuffer(lb *lensBuffer) {
 	lensBufferPool.Put(lb)
 }
 
-func getInmemoryBlock() *inmemoryBlock {
-	v := ibPool.Get()
-	if v == nil {
+func newInmemoryBlock() *inmemoryBlock {
 	return &inmemoryBlock{}
-	}
-	return v.(*inmemoryBlock)
 }
-
-func putInmemoryBlock(ib *inmemoryBlock) {
-	ib.Reset()
-	ibPool.Put(ib)
-}
-
-var ibPool sync.Pool

lib/mergeset/part_search.go

@@ -316,7 +316,7 @@ func (ps *partSearch) readInmemoryBlock(bh *blockHeader) (*inmemoryBlock, error)
 	ps.sb.lensData = bytesutil.Resize(ps.sb.lensData, int(bh.lensBlockSize))
 	ps.p.lensFile.MustReadAt(ps.sb.lensData, int64(bh.lensBlockOffset))
 
-	ib := getInmemoryBlock()
+	ib := newInmemoryBlock()
 	if err := ib.UnmarshalData(&ps.sb, bh.firstItem, bh.commonPrefix, bh.itemsCount, bh.marshalType); err != nil {
 		return nil, fmt.Errorf("cannot unmarshal storage block with %d items: %w", bh.itemsCount, err)
 	}

lib/mergeset/table.go

@@ -182,16 +182,15 @@ func (ris *rawItemsShard) addItems(tb *Table, items [][]byte) error {
 	ris.mu.Lock()
 	ibs := ris.ibs
 	if len(ibs) == 0 {
-		ib := getInmemoryBlock()
+		ib := newInmemoryBlock()
 		ibs = append(ibs, ib)
 		ris.ibs = ibs
 	}
 	ib := ibs[len(ibs)-1]
 	for _, item := range items {
 		if !ib.Add(item) {
-			ib = getInmemoryBlock()
+			ib = newInmemoryBlock()
 			if !ib.Add(item) {
-				putInmemoryBlock(ib)
 				err = fmt.Errorf("cannot insert an item %q into an empty inmemoryBlock; it looks like the item is too large? len(item)=%d", item, len(item))
 				break
 			}
@@ -675,13 +674,13 @@ func (tb *Table) mergeRawItemsBlocks(blocksToMerge []*inmemoryBlock) {
 func (tb *Table) mergeInmemoryBlocks(blocksToMerge []*inmemoryBlock) *partWrapper {
 	// Convert blocksToMerge into inmemoryPart's
 	mps := make([]*inmemoryPart, 0, len(blocksToMerge))
-	for _, ib := range blocksToMerge {
+	for i, ib := range blocksToMerge {
 		if len(ib.items) == 0 {
 			continue
 		}
 		mp := getInmemoryPart()
 		mp.Init(ib)
-		putInmemoryBlock(ib)
+		blocksToMerge[i] = nil
 		mps = append(mps, mp)
 	}
 	if len(mps) == 0 {