diff --git a/lib/store/base/buffer_readwriter.go b/lib/store/base/buffer_readwriter.go
index 67aa5191e..1b009f994 100644
--- a/lib/store/base/buffer_readwriter.go
+++ b/lib/store/base/buffer_readwriter.go
@@ -17,54 +17,110 @@ package base
 import (
 	"fmt"
 	"io"
-
-	"github.com/aws/aws-sdk-go/aws"
+	"sync"
+	"sync/atomic"
 )
 
 var _ FileReadWriter = &BufferReadWriter{}
 
-// BufferReadWriter implements FileReadWriter interface for in-memory buffering.
+// BufferReadWriter implements FileReadWriter for in-memory buffering.
+//
+// Pre-sizing (size > 0) allows concurrent WriteAt calls to non-overlapping
+// ranges to run in parallel. Without pre-sizing, each write that grows the
+// buffer is serialized.
+//
+// Bytes, Size, Write, Read, ReadAt, and Seek must not be called concurrently
+// with each other or with WriteAt.
 type BufferReadWriter struct {
-	buf    *aws.WriteAtBuffer
-	offset int64
+	mu      sync.RWMutex
+	buf     []byte
+	written atomic.Int64
+	offset  int64
 }
 
-// NewBufferReadWriter creates a new BufferReadWriter with an initial capacity of size bytes.
+// NewBufferReadWriter creates a new BufferReadWriter pre-allocated to size bytes.
+// Pass the exact blob size when known so concurrent WriteAt calls for
+// non-overlapping shard ranges can run in parallel without writer serialization.
 func NewBufferReadWriter(size uint64) *BufferReadWriter {
-	bytesSlice := make([]byte, 0, size)
-	buf := aws.NewWriteAtBuffer(bytesSlice)
-	// Although this is default, this is explicitly set to notify that we are reserving
-	// only as much capacity as needed
-	buf.GrowthCoeff = 1
-
-	return &BufferReadWriter{
-		buf:    buf,
-		offset: 0,
-	}
+	return &BufferReadWriter{buf: make([]byte, size)}
 }
 
-// Write implements io.Writer by using WriteAt with current write offset.
+// Write implements io.Writer using the current sequential write offset.
 func (b *BufferReadWriter) Write(p []byte) (n int, err error) {
-	n, err = b.buf.WriteAt(p, b.offset)
+	n, err = b.WriteAt(p, b.offset)
 	b.offset += int64(n)
 	return n, err
 }
 
-// WriteAt implements io.WriterAt for parallel writes.
-func (b *BufferReadWriter) WriteAt(p []byte, off int64) (n int, err error) {
+// WriteAt implements io.WriterAt.
+//
+// Fast path (off+len(p) within pre-allocated buffer): multiple goroutines may
+// call WriteAt concurrently, provided their byte ranges do not overlap.
+//
+// Slow path (write extends beyond current buffer): acquires an exclusive lock
+// only to grow the buffer, then copies under a shared read lock so other
+// writers can proceed in parallel.
+func (b *BufferReadWriter) WriteAt(p []byte, off int64) (int, error) {
 	if off < 0 {
 		return 0, fmt.Errorf("negative offset")
 	}
-	return b.buf.WriteAt(p, off)
+	if len(p) == 0 {
+		return 0, nil
+	}
+	end := off + int64(len(p))
+	if end < off {
+		return 0, fmt.Errorf("write at offset %d length %d overflows int64", off, len(p))
+	}
+
+	// fast path
+	if n, shouldGrowBuffer := b.updateBuffer(p, off, end); !shouldGrowBuffer {
+		return n, nil
+	}
+
+	// slow path: grow, then call updateBuffer again
+	b.growBuffer(end)
+	n, _ := b.updateBuffer(p, off, end)
+	return n, nil
+}
+
+// growBuffer expands b.buf to at least size bytes. It is a no-op when the
+// buffer is already large enough.
+func (b *BufferReadWriter) growBuffer(size int64) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if size <= int64(len(b.buf)) {
+		return
+	}
+	grown := make([]byte, size)
+	copy(grown, b.buf)
+	b.buf = grown
+}
+
+// updateBuffer copies p into [off, end).
+// Returns (-1, true) if end exceeds the buffer, causing WriteAt to take the slow path.
+func (b *BufferReadWriter) updateBuffer(p []byte, off, end int64) (int, bool) {
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	if end <= int64(len(b.buf)) {
+		n := copy(b.buf[off:], p)
+		for {
+			cur := b.written.Load()
+			if end <= cur || b.written.CompareAndSwap(cur, end) {
+				break
+			}
+		}
+		return n, false
+	}
+	return -1, true
 }
 
 // Read implements io.Reader for sequential reads.
 func (b *BufferReadWriter) Read(p []byte) (n int, err error) {
-	bufBytes := b.buf.Bytes()
-	if b.offset >= int64(len(bufBytes)) {
+	written := b.written.Load()
+	if b.offset >= written {
 		return 0, io.EOF
 	}
-	n = copy(p, bufBytes[b.offset:])
+	n = copy(p, b.buf[b.offset:written])
 	b.offset += int64(n)
 	if n < len(p) {
 		err = io.EOF
@@ -77,11 +133,14 @@ func (b *BufferReadWriter) ReadAt(p []byte, off int64) (n int, err error) {
 	if off < 0 {
 		return 0, fmt.Errorf("negative offset")
 	}
-	bufBytes := b.buf.Bytes()
-	if off >= int64(len(bufBytes)) {
+	b.mu.RLock()
+	buf := b.buf
+	written := b.written.Load()
+	b.mu.RUnlock()
+	if off >= written {
 		return 0, io.EOF
 	}
-	n = copy(p, bufBytes[off:])
+	n = copy(p, buf[off:written])
 	if n < len(p) {
 		err = io.EOF
 	}
@@ -91,23 +150,19 @@ func (b *BufferReadWriter) ReadAt(p []byte, off int64) (n int, err error) {
 // Seek implements io.Seeker.
 func (b *BufferReadWriter) Seek(offset int64, whence int) (int64, error) {
 	var newOffset int64
-	bufSize := int64(len(b.buf.Bytes()))
-
 	switch whence {
 	case io.SeekStart:
 		newOffset = offset
 	case io.SeekCurrent:
 		newOffset = b.offset + offset
 	case io.SeekEnd:
-		newOffset = bufSize + offset
+		newOffset = b.written.Load() + offset
 	default:
 		return 0, fmt.Errorf("invalid whence: %d", whence)
 	}
-
 	if newOffset < 0 {
 		return 0, fmt.Errorf("negative position: %d", newOffset)
 	}
-
 	b.offset = newOffset
 	return newOffset, nil
 }
@@ -117,10 +172,8 @@ func (b *BufferReadWriter) Close() error {
 	return nil
 }
 
-// Size returns the size of the buffer
-func (b *BufferReadWriter) Size() int64 {
-	return int64(len(b.buf.Bytes()))
-}
+// Size returns the largest end offset written so far.
+func (b *BufferReadWriter) Size() int64 { return b.written.Load() }
 
 // Cancel is no-op
 func (b *BufferReadWriter) Cancel() error {
@@ -132,7 +185,8 @@ func (b *BufferReadWriter) Commit() error {
 	return nil
 }
 
-// Bytes returns the full buffer
+// Bytes returns the buffer up to the highest offset written so far.
+// Any gaps between writes are zero-filled.
 func (b *BufferReadWriter) Bytes() []byte {
-	return b.buf.Bytes()
+	return b.buf[:b.written.Load()]
 }
diff --git a/lib/store/base/buffer_readwriter_test.go b/lib/store/base/buffer_readwriter_test.go
index cfcf7c5bf..7316290ff 100644
--- a/lib/store/base/buffer_readwriter_test.go
+++ b/lib/store/base/buffer_readwriter_test.go
@@ -15,7 +15,11 @@
 package base
 
 import (
+	"errors"
+	"fmt"
 	"io"
+	"runtime"
+	"sync"
 	"testing"
 	"testing/iotest"
 
@@ -49,6 +53,7 @@ func TestBufferReadWriter_Write(t *testing.T) {
 			}
 
 			assert.Equal(t, tt.expectedSize, buf.Size())
+			assert.Equal(t, tt.expectedResult, buf.Bytes())
 		})
 	}
 }
@@ -111,6 +116,9 @@ func TestBufferReadWriter_WriteAt(t *testing.T) {
 			}
 
 			assert.Equal(t, tt.expectedSize, buf.Size())
+			if tt.expectedResult != nil {
+				assert.Equal(t, tt.expectedResult, buf.Bytes())
+			}
 		})
 	}
 }
@@ -308,3 +316,141 @@ func TestBufferReadWriter_TestReader(t *testing.T) {
 	err = iotest.TestReader(buf, content)
 	require.NoError(t, err)
 }
+
+func TestBufferReadWriter_ConcurrentWriteAt(t *testing.T) {
+	const numShards, shardSize = 10, 1024
+	totalSize := uint64(numShards * shardSize)
+
+	tests := []struct {
+		name     string
+		initSize uint64
+	}{
+		{"presized", totalSize},
+		{"half_presized", totalSize / 2},
+		{"dynamic", 0},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			data := make([]byte, totalSize)
+			for i := range data {
+				data[i] = byte(i % 256)
+			}
+
+			buf := NewBufferReadWriter(tt.initSize)
+			errs := make([]error, numShards)
+			var wg sync.WaitGroup
+			for i := 0; i < numShards; i++ {
+				wg.Add(1)
+				go func(shard int) {
+					defer wg.Done()
+					off := shard * shardSize
+					_, errs[shard] = buf.WriteAt(data[off:off+shardSize], int64(off))
+				}(i)
+			}
+			wg.Wait()
+
+			require.NoError(t, errors.Join(errs...))
+			assert.Equal(t, data, buf.Bytes())
+		})
+	}
+}
+
+func TestBufferReadWriter_WriteAtEmpty(t *testing.T) {
+	buf := NewBufferReadWriter(0)
+	n, err := buf.WriteAt(nil, 1<<30)
+	require.NoError(t, err)
+	assert.Equal(t, 0, n)
+	assert.Equal(t, int64(0), buf.Size())
+	assert.Empty(t, buf.Bytes())
+}
+
+func totalMutexContentions() int64 {
+	size := 1024
+	for {
+		records := make([]runtime.BlockProfileRecord, size)
+		n, ok := runtime.MutexProfile(records)
+		if ok {
+			var total int64
+			for i := 0; i < n; i++ {
+				total += records[i].Count
+			}
+			return total
+		}
+		size = n + 64
+	}
+}
+
+// benchmarkWriteAt is the shared helper for all WriteAt benchmarks.
+// numShards goroutines each write a non-overlapping 4 MiB shard concurrently,
+// initSize controls the buffer's initial allocation:
+//   - initSize == totalSize: pre-sized fast path (production case)
+//   - initSize == 0:         dynamic growth (sequential / wrong-size case)
+//   - initSize == totalSize/2: partial pre-allocation, triggers growth mid-download
+func benchmarkWriteAt(b *testing.B, numShards int, initSize uint64) {
+	b.Helper()
+	const shardSize = 4 * 1024 * 1024
+	totalSize := uint64(numShards) * shardSize
+
+	shards := make([][]byte, numShards)
+	for i := range shards {
+		shards[i] = make([]byte, shardSize)
+		for j := range shards[i] {
+			shards[i][j] = byte(i)
+		}
+	}
+
+	prev := runtime.SetMutexProfileFraction(1)
+	defer runtime.SetMutexProfileFraction(prev)
+	startContentions := totalMutexContentions()
+	b.SetBytes(int64(totalSize))
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	errs := make([]error, numShards)
+	for b.Loop() {
+		buf := NewBufferReadWriter(initSize)
+		var wg sync.WaitGroup
+		for shard := 0; shard < numShards; shard++ {
+			wg.Add(1)
+			go func(s int) {
+				defer wg.Done()
+				_, errs[s] = buf.WriteAt(shards[s], int64(s)*shardSize)
+			}(shard)
+		}
+		wg.Wait()
+		if err := errors.Join(errs...); err != nil {
+			b.Fatal(err)
+		}
+	}
+
+	b.StopTimer()
+	if b.N > 0 {
+		b.ReportMetric(float64(totalMutexContentions()-startContentions)/float64(b.N), "mutex-contentions/op")
+	}
+}
+
+// BenchmarkBufferReadWriter_WriteAt exercises three buffer initialization
+// strategies × three shard counts.
+func BenchmarkBufferReadWriter_WriteAt(b *testing.B) {
+	const shardSize = 4 * 1024 * 1024
+	cases := []struct {
+		label    string
+		initFunc func(total uint64) uint64
+	}{
+		{"presized", func(total uint64) uint64 { return total }},          // production fast path
+		{"half_presized", func(total uint64) uint64 { return total / 2 }}, // partial pre-alloc, growth needed
+		{"dynamic", func(total uint64) uint64 { return 0 }},               // no pre-alloc, always grows
+	}
+	shardCounts := []int{1, 4, 10}
+
+	for _, tc := range cases {
+		for _, numShards := range shardCounts {
+			totalSize := uint64(numShards) * shardSize
+			initSize := tc.initFunc(totalSize)
+			b.Run(fmt.Sprintf("%s_%d_shards", tc.label, numShards), func(b *testing.B) {
+				benchmarkWriteAt(b, numShards, initSize)
+			})
+		}
+	}
+}