diff --git a/sstable/colblk_writer.go b/sstable/colblk_writer.go
index eae0e92c8c..afcbe9a68d 100644
--- a/sstable/colblk_writer.go
+++ b/sstable/colblk_writer.go
@@ -1090,7 +1090,7 @@ func shouldFlushWithoutLatestKV(
 func (w *RawColumnWriter) copyDataBlocks(
 	ctx context.Context, blocks []indexEntry, rh objstorage.ReadHandle,
 ) error {
-	buf := make([]byte, 0, 256<<10)
+	const readSizeTarget = 256 << 10
 	readAndFlushBlocks := func(firstBlockIdx, lastBlockIdx int) error {
 		if firstBlockIdx > lastBlockIdx {
 			panic("pebble: readAndFlushBlocks called with invalid block range")
@@ -1103,9 +1103,9 @@ func (w *RawColumnWriter) copyDataBlocks(
 		// blocks in one request.
 		lastBH := blocks[lastBlockIdx].bh
 		blocksToReadLen := lastBH.Offset + lastBH.Length + block.TrailerLen - blocks[firstBlockIdx].bh.Offset
-		if blocksToReadLen > uint64(cap(buf)) {
-			buf = make([]byte, 0, blocksToReadLen)
-		}
+		// We need to create a new buffer for each read, as w.enqueuePhysicalBlock passes
+		// a pointer to the buffer to the write queue.
+		buf := make([]byte, 0, blocksToReadLen)
 		if err := rh.ReadAt(ctx, buf[:blocksToReadLen], int64(blocks[firstBlockIdx].bh.Offset)); err != nil {
 			return err
 		}
@@ -1120,8 +1120,8 @@ func (w *RawColumnWriter) copyDataBlocks(
 		}
 		return nil
 	}
-	// Iterate through blocks until we have enough to fill cap(buf). When we have more than
-	// one block in blocksToRead and adding the next block would exceed the buffer capacity,
+	// Iterate through blocks until we have enough to fill readSizeTarget. When we have more than
+	// one block in blocksToRead and adding the next block would exceed the target buffer capacity,
 	// we read and flush existing blocks in blocksToRead. This allows us to read as many
 	// blocks in one IO request as possible, while still utilizing the write queue in this
 	// writer.
@@ -1133,7 +1133,7 @@ func (w *RawColumnWriter) copyDataBlocks(
 		start := i
 		// Note the i++ in the initializing condition; this means we will always flush at least
 		// one block.
-		for i++; i < len(blocks) && (blocks[i].bh.Length+blocks[i].bh.Offset+block.TrailerLen-blocks[start].bh.Offset) <= uint64(cap(buf)); i++ {
+		for i++; i < len(blocks) && (blocks[i].bh.Length+blocks[i].bh.Offset+block.TrailerLen-blocks[start].bh.Offset) <= uint64(readSizeTarget); i++ {
 		}
 		// i points to one index past the last block we want to read.
 		if err := readAndFlushBlocks(start, i-1); err != nil {
diff --git a/sstable/copier_test.go b/sstable/copier_test.go
index 31759a513d..f5b7ce3724 100644
--- a/sstable/copier_test.go
+++ b/sstable/copier_test.go
@@ -23,7 +23,10 @@ import (
 
 func TestCopySpan(t *testing.T) {
 	fs := vfs.NewMem()
-	blockCache := cache.New(1 << 20 /* 1 MB */)
+	blockCache := cache.New(2 << 20 /* 1 MB */)
+	cacheID := cache.ID(1)
+	fileNameToNum := make(map[string]base.FileNum)
+	nextFileNum := base.FileNum(1)
 	defer blockCache.Unref()
 
 	keySchema := colblk.DefaultKeySchema(testkeys.Comparer, 16)
@@ -35,6 +38,8 @@ func TestCopySpan(t *testing.T) {
 			if err != nil {
 				return err.Error()
 			}
+			fileNameToNum[d.CmdArgs[0].Key] = nextFileNum
+			nextFileNum++
 			tableFormat := TableFormatMax
 			blockSize := 1
 			var indexBlockSize int
@@ -92,15 +97,28 @@ func TestCopySpan(t *testing.T) {
 			if err != nil {
 				return err.Error()
 			}
-			r, err := NewReader(context.TODO(), readable, ReaderOptions{
+			var start, end []byte
+			for _, arg := range d.CmdArgs[1:] {
+				switch arg.Key {
+				case "start":
+					start = []byte(arg.FirstVal(t))
+				case "end":
+					end = []byte(arg.FirstVal(t))
+				}
+			}
+			rOpts := ReaderOptions{
 				Comparer:   testkeys.Comparer,
 				KeySchemas: KeySchemas{keySchema.Name: &keySchema},
-			})
+			}
+			rOpts.internal.CacheOpts.Cache = blockCache
+			rOpts.internal.CacheOpts.CacheID = cacheID
+			rOpts.internal.CacheOpts.FileNum = base.DiskFileNum(fileNameToNum[d.CmdArgs[0].Key])
+			r, err := NewReader(context.TODO(), readable, rOpts)
 			defer r.Close()
 			if err != nil {
 				return err.Error()
 			}
-			iter, err := r.NewIter(block.NoTransforms, nil, nil)
+			iter, err := r.NewIter(block.NoTransforms, start, end)
 			if err != nil {
 				return err.Error()
 			}
@@ -126,6 +144,8 @@ func TestCopySpan(t *testing.T) {
 				return err.Error()
 			}
 			writable := objstorageprovider.NewFileWritable(output)
+			fileNameToNum[outputFile] = nextFileNum
+			nextFileNum++
 
 			f, err := fs.Open(inputFile)
 			if err != nil {
@@ -140,6 +160,8 @@ func TestCopySpan(t *testing.T) {
 				KeySchemas: KeySchemas{keySchema.Name: &keySchema},
 			}
 			rOpts.internal.CacheOpts.Cache = blockCache
+			rOpts.internal.CacheOpts.CacheID = cacheID
+			rOpts.internal.CacheOpts.FileNum = base.DiskFileNum(fileNameToNum[inputFile])
 			r, err := NewReader(context.TODO(), readable, rOpts)
 			if err != nil {
 				return err.Error()
diff --git a/sstable/testdata/copy_span b/sstable/testdata/copy_span
index 099a9b06a9..16da2f59f4 100644
--- a/sstable/testdata/copy_span
+++ b/sstable/testdata/copy_span
@@ -92,10 +92,8 @@ c.SET.4:baz
 d.SET.5:foobar
 ----
 
-iter test3
+iter test3 start=c
 ----
-a#0,SET: foo
-b#0,SET: bar
 c#0,SET: baz
 d#0,SET: foobar
 
@@ -134,18 +132,10 @@ i.SET.5:foo
 j.SET.5:foo
 ----
 
-iter test32
+iter test32 start=c end=e
 ----
-a#0,SET: foo
-b#0,SET: bar
 c#0,SET: baz
 d#0,SET: foobar
-e#0,SET: foo
-f#0,SET: foo
-g#0,SET: foo
-h#0,SET: foo
-i#0,SET: foo
-j#0,SET: foo
 
 copy-span test32 test33 b.SET.10 cc.SET.0
 ----