diff --git a/block/simple_committed_block_index.go b/block/simple_committed_block_index.go
index 211fcdd28..d43fc8064 100644
--- a/block/simple_committed_block_index.go
+++ b/block/simple_committed_block_index.go
@@ -18,9 +18,8 @@
 type simpleCommittedBlockIndex struct {
 	dirname string
 
-	mu          sync.Mutex
-	indexBlocks map[PhysicalBlockID]bool
-	merged      packindex.Merged
+	mu     sync.Mutex
+	merged packindex.Merged
 }
 
 func (b *simpleCommittedBlockIndex) getBlock(blockID string) (Info, error) {
@@ -41,32 +40,78 @@ func (b *simpleCommittedBlockIndex) hasIndexBlockID(indexBlockID PhysicalBlockID
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
-	return b.indexBlocks[indexBlockID], nil
+	_, err := os.Stat(b.indexBlockPath(indexBlockID))
+	if err == nil {
+		return true, nil
+	}
+	if os.IsNotExist(err) {
+		return false, nil
+	}
+
+	return false, err
 }
 
-func (b *simpleCommittedBlockIndex) addBlock(indexBlockID PhysicalBlockID, data []byte, use bool) error {
-	fullPath := filepath.Join(b.dirname, string(indexBlockID+simpleIndexSuffix))
+func (b *simpleCommittedBlockIndex) indexBlockPath(indexBlockID PhysicalBlockID) string {
+	return filepath.Join(b.dirname, string(indexBlockID+simpleIndexSuffix))
+}
 
-	if err := ioutil.WriteFile(fullPath, data, 0600); err != nil {
+func (b *simpleCommittedBlockIndex) addBlockToCache(indexBlockID PhysicalBlockID, data []byte) error {
+	exists, err := b.hasIndexBlockID(indexBlockID)
+	if err != nil {
 		return err
 	}
 
-	b.mu.Lock()
-	defer b.mu.Unlock()
+	if exists {
+		return nil
+	}
 
-	b.indexBlocks[indexBlockID] = true
+	// write to a temp file to avoid race where two processes are writing at the same time.
+	tf, err := ioutil.TempFile(b.dirname, "index")
+	if err != nil {
+		return fmt.Errorf("can't create tmp file: %v", err)
+	}
+	defer os.Remove(tf.Name()) //nolint:errcheck
+
+	if _, err := tf.Write(data); err != nil {
+		return fmt.Errorf("can't write to temp file: %v", err)
+	}
+	if err := tf.Close(); err != nil {
+		return fmt.Errorf("can't close tmp file")
+	}
+
+	// rename() is atomic, so one process will succeed, but the other will fail
+	if err := os.Rename(tf.Name(), b.indexBlockPath(indexBlockID)); err != nil {
+		// verify that the block exists
+		exists, err := b.hasIndexBlockID(indexBlockID)
+		if err != nil {
+			return err
+		}
+		if !exists {
+			return fmt.Errorf("unsuccessful index write of block %q", indexBlockID)
+		}
+	}
+
+	return nil
+}
+
+func (b *simpleCommittedBlockIndex) addBlock(indexBlockID PhysicalBlockID, data []byte, use bool) error {
+	if err := b.addBlockToCache(indexBlockID, data); err != nil {
+		return err
+	}
 
 	if !use {
 		return nil
 	}
 
-	ndx, err := b.openIndex(fullPath)
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	ndx, err := b.openIndex(b.indexBlockPath(indexBlockID))
 	if err != nil {
-		return fmt.Errorf("unable to open pack index %q: %v", fullPath, err)
+		return fmt.Errorf("unable to open pack index %q: %v", indexBlockID, err)
 	}
 
 	b.merged = append(b.merged, ndx)
-
 	return nil
 }
 
@@ -80,7 +125,6 @@ func (b *simpleCommittedBlockIndex) listBlocks(prefix string, cb func(i Info) er
 
 func (b *simpleCommittedBlockIndex) openIndex(fullpath string) (packindex.Index, error) {
 	f, err := mmap.Open(fullpath)
-	//f, err := os.Open(fullpath)
 	if err != nil {
 		return nil, err
 	}
@@ -92,7 +136,6 @@ func (b *simpleCommittedBlockIndex) use(packBlockIDs []PhysicalBlockID) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
-	newIndexes := map[PhysicalBlockID]bool{}
 	var newMerged packindex.Merged
 	defer func() {
 		newMerged.Close() //nolint:errcheck
@@ -105,10 +148,8 @@ func (b *simpleCommittedBlockIndex) use(packBlockIDs []PhysicalBlockID) error {
 		}
 
 		log.Printf("opened %v with %v entries", fullpath, ndx.EntryCount())
-		newIndexes[e] = true
 		newMerged = append(newMerged, ndx)
 	}
-	b.indexBlocks = newIndexes
 	b.merged = newMerged
 	newMerged = nil
 	return nil
@@ -118,8 +159,7 @@ func newSimpleCommittedBlockIndex(dirname string) (committedBlockIndex, error) {
 	_ = os.MkdirAll(dirname, 0700)
 
 	s := &simpleCommittedBlockIndex{
-		dirname:     dirname,
-		indexBlocks: map[PhysicalBlockID]bool{},
+		dirname: dirname,
 	}
 	return s, nil
 }
diff --git a/cli/command_snapshot_estimate.go b/cli/command_snapshot_estimate.go
index 80b4d5c72..ff6493568 100644
--- a/cli/command_snapshot_estimate.go
+++ b/cli/command_snapshot_estimate.go
@@ -3,13 +3,12 @@
 import (
 	"context"
 	"fmt"
+	"os"
 	"path/filepath"
 	"time"
 
 	"github.com/kopia/kopia/repo"
 
-	"github.com/rs/zerolog/log"
-
 	"github.com/kopia/kopia/fs"
 	"github.com/kopia/kopia/internal/units"
 	"github.com/kopia/kopia/snapshot"
@@ -121,7 +120,7 @@ func estimate(ctx context.Context, relativePath string, entry fs.Entry, pol *sna
 	switch entry := entry.(type) {
 	case fs.Directory:
 		if !*snapshotEstimateQuiet {
-			log.Printf("Scanning %q...\n", relativePath)
+			fmt.Fprintln(os.Stderr, "Scanning", relativePath)
 		}
 		children, err := entry.Readdir(ctx)
 		if err != nil {