diff --git a/backend/posix/data_io_linux.go b/backend/posix/data_io_linux.go
index 4c1b552c..358aefa7 100644
--- a/backend/posix/data_io_linux.go
+++ b/backend/posix/data_io_linux.go
@@ -26,6 +26,8 @@ import (
 	"syscall"
 
 	"github.com/versity/versitygw/backend"
+	"github.com/versity/versitygw/debuglogger"
+	"golang.org/x/sys/unix"
 )
 
 // openDataRead opens object data for reading and applies O_DIRECT when
@@ -49,144 +51,177 @@ func openDataRead(name string, useODirect bool) (*os.File, error) {
 	return f, nil
 }
 
-func buildGetObjectBody(f *os.File, path string, startOffset, length, objSize int64, useODirect bool, readBufferSize int) (io.ReadCloser, error) {
+func buildGetObjectBody(f *os.File, _ string, startOffset, length, objSize int64, useODirect bool, readBufferSize int) (io.ReadCloser, error) {
+	// openDataRead may have silently opened without O_DIRECT.
+	if useODirect && isODirectFile(f) {
+		return newODirectReader(f, startOffset, length, readBufferSize), nil
+	}
+
 	if startOffset == 0 && length == objSize {
-		if useODirect {
-			return newODirectReadFallbackFile(path, f, true), nil
-		}
 		return f, nil
 	}
 
-	if !useODirect {
-		rdr := io.NewSectionReader(f, startOffset, length)
-		return withReadBufferSize(&backend.FileSectionReadCloser{R: rdr, F: f}, readBufferSize), nil
+	rdr := io.NewSectionReader(f, startOffset, length)
+	return withReadBufferSize(&backend.FileSectionReadCloser{R: rdr, F: f}, readBufferSize), nil
+}
+
+func isODirectFile(f *os.File) bool {
+	flags, err := unix.FcntlInt(f.Fd(), unix.F_GETFL, 0)
+	return err == nil && flags&unix.O_DIRECT != 0
+}
+
+var odirectReadBufPool sync.Pool
+
+// odirectReader serves [off, end) of f using block-aligned preads into an
+// aligned buffer, as O_DIRECT requires for offset, length, and memory.
+type odirectReader struct {
+	mu     sync.Mutex
+	f      *os.File
+	direct bool
+	off    int64
+	end    int64
+	buf    []byte
+	bufOff int64
+	bufLen int
+}
+
+func newODirectReader(f *os.File, off, length int64, bufSize int) *odirectReader {
+	size := max(odirectAlign, (bufSize+odirectAlign-1)/odirectAlign*odirectAlign)
+	buf, ok := odirectReadBufPool.Get().(*[]byte)
+	if !ok || len(*buf) != size {
+		b := alignedBuffer(size)
+		buf = &b
 	}
-
-	rf := newODirectReadFallbackFile(path, f, true)
-
-	if _, err := rf.Seek(startOffset, io.SeekStart); err != nil {
-		_ = rf.Close()
-		return nil, fmt.Errorf("seek range start: %w", err)
-	}
-
-	return withReadBufferSize(&readerWithCloser{r: io.LimitReader(rf, length), c: rf}, readBufferSize), nil
-}
-
-type readerWithCloser struct {
-	r io.Reader
-	c io.Closer
-}
-
-func (r *readerWithCloser) Read(p []byte) (int, error) {
-	return r.r.Read(p)
-}
-
-func (r *readerWithCloser) Close() error {
-	return r.c.Close()
-}
-
-type odirectReadFallbackFile struct {
-	mu         sync.Mutex
-	path       string
-	f          *os.File
-	useODirect bool
-}
-
-func newODirectReadFallbackFile(path string, f *os.File, useODirect bool) *odirectReadFallbackFile {
-	return &odirectReadFallbackFile{
-		path:       path,
-		f:          f,
-		useODirect: useODirect,
+	return &odirectReader{
+		f:      f,
+		direct: true,
+		off:    off,
+		end:    off + length,
+		buf:    *buf,
 	}
 }
 
-func (r *odirectReadFallbackFile) Read(p []byte) (int, error) {
+func (r *odirectReader) Read(p []byte) (int, error) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 
-	n, err := r.f.Read(p)
-	if err != nil && n == 0 && r.useODirect && isODirectRuntimeFallbackErr(err) {
-		if fallbackErr := r.switchToBufferedAtCurrentOffsetLocked(); fallbackErr != nil {
-			return 0, fallbackErr
+	chunk, err := r.chunkLocked()
+	if err != nil {
+		return 0, err
+	}
+	n := copy(p, chunk)
+	r.off += int64(n)
+	return n, nil
+}
+
+func (r *odirectReader) WriteTo(w io.Writer) (int64, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	var written int64
+	for {
+		chunk, err := r.chunkLocked()
+		if err == io.EOF {
+			return written, nil
+		}
+		if err != nil {
+			return written, err
+		}
+		n, err := w.Write(chunk)
+		r.off += int64(n)
+		written += int64(n)
+		if err != nil {
+			return written, err
 		}
-
-		return r.f.Read(p)
 	}
-
-	return n, err
 }
 
-func (r *odirectReadFallbackFile) WriteTo(w io.Writer) (int64, error) {
-	r.mu.Lock()
-	useODirect := r.useODirect
-	f := r.f
-	r.mu.Unlock()
-
-	if !useODirect {
-		return io.Copy(w, &onlyRead{r})
-	}
-
-	writerTo, ok := interface{}(f).(io.WriterTo)
-	if !ok {
-		return io.Copy(w, &onlyRead{r})
-	}
-
-	n, err := writerTo.WriteTo(w)
-	if err == nil || !isODirectRuntimeFallbackErr(err) {
-		return n, err
-	}
-
-	r.mu.Lock()
-	fallbackErr := r.switchToBufferedAtCurrentOffsetLocked()
-	r.mu.Unlock()
-	if fallbackErr != nil {
-		return n, fallbackErr
-	}
-
-	m, err := io.Copy(w, &onlyRead{r})
-	return n + m, err
-}
-
-func (r *odirectReadFallbackFile) Seek(offset int64, whence int) (int64, error) {
+func (r *odirectReader) Close() error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	return r.f.Seek(offset, whence)
-}
 
-func (r *odirectReadFallbackFile) Close() error {
-	r.mu.Lock()
-	defer r.mu.Unlock()
+	if r.buf != nil {
+		buf := r.buf
+		odirectReadBufPool.Put(&buf)
+		r.buf = nil
+	}
 	return r.f.Close()
 }
 
-func (r *odirectReadFallbackFile) switchToBufferedAtOffset(offset int64) error {
-	fd := strconv.Itoa(int(r.f.Fd()))
-	bf, openErr := os.Open(filepath.Join(procfddir, fd))
-	if openErr != nil {
-		return openErr
+// chunkLocked returns the buffered bytes starting at r.off, reading more if needed.
+func (r *odirectReader) chunkLocked() ([]byte, error) {
+	if r.buf == nil {
+		return nil, os.ErrClosed
+	}
+	if r.off >= r.end {
+		return nil, io.EOF
+	}
+	if r.off < r.bufOff || r.off >= r.bufOff+int64(r.bufLen) {
+		if err := r.fillLocked(); err != nil {
+			return nil, err
+		}
 	}
 
-	if _, seekErr := bf.Seek(offset, io.SeekStart); seekErr != nil {
+	stop := int64(r.bufLen)
+	if rem := r.end - r.bufOff; rem < stop {
+		stop = rem
+	}
+	return r.buf[r.off-r.bufOff : stop], nil
+}
+
+func (r *odirectReader) fillLocked() error {
+	alignedOff := r.off &^ (odirectAlign - 1)
+	buf := r.buf
+	if need := (r.end - alignedOff + odirectAlign - 1) &^ (odirectAlign - 1); need < int64(len(buf)) {
+		buf = buf[:int(need)]
+	}
+	n, err := pread(r.f, buf, alignedOff)
+	if n == 0 && r.direct && isODirectRuntimeFallbackErr(err) {
+		debuglogger.Logf("O_DIRECT read of %s failed at offset %d, falling back to buffered I/O: %v", r.f.Name(), alignedOff, err)
+		if ferr := r.switchToBufferedLocked(); ferr != nil {
+			return fmt.Errorf("reopen object in buffered mode after O_DIRECT read failure: %w", ferr)
+		}
+		n, err = pread(r.f, buf, alignedOff)
+	}
+
+	r.bufOff, r.bufLen = alignedOff, n
+	if r.off < alignedOff+int64(n) {
+		return nil
+	}
+	if err == nil || err == io.EOF {
+		return io.ErrUnexpectedEOF
+	}
+	return err
+}
+
+func (r *odirectReader) switchToBufferedLocked() error {
+	bf, err := os.Open(filepath.Join(procfddir, strconv.Itoa(int(r.f.Fd()))))
+	if err != nil {
+		return err
+	}
+	if err := r.f.Close(); err != nil {
 		_ = bf.Close()
-		return seekErr
+		return err
 	}
-
-	if closeErr := r.f.Close(); closeErr != nil {
-		_ = bf.Close()
-		return closeErr
-	}
-
 	r.f = bf
-	r.useODirect = false
-
+	r.direct = false
 	return nil
 }
 
-func (r *odirectReadFallbackFile) switchToBufferedAtCurrentOffsetLocked() error {
-	offset, seekErr := r.f.Seek(0, io.SeekCurrent)
-	if seekErr != nil {
-		return seekErr
+// pread issues a single pread; os.File.ReadAt would loop into an unaligned
+// offset after a short read at EOF, which O_DIRECT can reject.
+func pread(f *os.File, b []byte, off int64) (int, error) {
+	for {
+		n, err := unix.Pread(int(f.Fd()), b, off)
+		if err == unix.EINTR {
+			continue
+		}
+		if err != nil {
+			return 0, err
+		}
+		if n == 0 {
+			return 0, io.EOF
+		}
+		return n, nil
 	}
-
-	return r.switchToBufferedAtOffset(offset)
 }
diff --git a/backend/posix/data_io_linux_test.go b/backend/posix/data_io_linux_test.go
new file mode 100644
index 00000000..b950a59f
--- /dev/null
+++ b/backend/posix/data_io_linux_test.go
@@ -0,0 +1,102 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//go:build linux
+
+package posix
+
+import (
+	"bytes"
+	"io"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestODirectReaderRanges(t *testing.T) {
+	const objSize = 3*1024*1024 + 777
+	data := make([]byte, objSize)
+	rand.New(rand.NewSource(3)).Read(data)
+
+	path := filepath.Join(t.TempDir(), "obj")
+	if err := os.WriteFile(path, data, 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	ranges := []struct{ off, length int64 }{
+		{0, objSize},
+		{0, 0},
+		{1, 1},
+		{4095, 2},
+		{123457, 1024*1024 + 3},
+		{objSize - 777, 777},
+		{objSize - 1, 1},
+	}
+
+	for _, direct := range []bool{false, true} {
+		for _, rg := range ranges {
+			for _, useWriteTo := range []bool{false, true} {
+				f, err := openDataRead(path, direct)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if direct && !isODirectFile(f) {
+					f.Close()
+					t.Skip("O_DIRECT not supported on temp filesystem")
+				}
+
+				r := newODirectReader(f, rg.off, rg.length, 64*1024+1)
+				var got bytes.Buffer
+				if useWriteTo {
+					_, err = r.WriteTo(&got)
+				} else {
+					_, err = io.Copy(&got, struct{ io.Reader }{r})
+				}
+				if err != nil {
+					t.Fatalf("direct=%v off=%d len=%d writeTo=%v: %v", direct, rg.off, rg.length, useWriteTo, err)
+				}
+				if !bytes.Equal(got.Bytes(), data[rg.off:rg.off+rg.length]) {
+					t.Fatalf("direct=%v off=%d len=%d writeTo=%v: content mismatch", direct, rg.off, rg.length, useWriteTo)
+				}
+				if direct && rg.off == 1 && rg.length == 1 && r.bufLen != odirectAlign {
+					t.Fatalf("small range read %d bytes, want %d", r.bufLen, odirectAlign)
+				}
+				if direct && !r.direct {
+					t.Fatalf("off=%d len=%d: fell back to buffered I/O", rg.off, rg.length)
+				}
+				if err := r.Close(); err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+	}
+}
+
+func TestODirectReaderTruncatedFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "obj")
+	if err := os.WriteFile(path, make([]byte, 1000), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	r := newODirectReader(f, 0, 2000, 4096)
+	defer r.Close()
+	if _, err := io.Copy(io.Discard, struct{ io.Reader }{r}); err != io.ErrUnexpectedEOF {
+		t.Fatalf("got %v, want %v", err, io.ErrUnexpectedEOF)
+	}
+}
diff --git a/backend/posix/otmpfile_common.go b/backend/posix/otmpfile_common.go
index 0530d9b0..37177a76 100644
--- a/backend/posix/otmpfile_common.go
+++ b/backend/posix/otmpfile_common.go
@@ -23,9 +23,28 @@ import (
 	"strconv"
 	"syscall"
 	"time"
+	"unsafe"
+
+	"github.com/versity/versitygw/debuglogger"
 )
 
-const odirectMinWriteAlign = 512
+// odirectAlign covers both 512e and 4Kn logical block sizes for O_DIRECT
+// offsets, lengths, and memory.
+const odirectAlign = 4096
+
+// alignedBuffer returns a size-byte slice whose first byte is odirectAlign aligned.
+func alignedBuffer(size int) []byte {
+	b := make([]byte, size+odirectAlign)
+	off := 0
+	if rem := int(uintptr(unsafe.Pointer(&b[0])) % odirectAlign); rem != 0 {
+		off = odirectAlign - rem
+	}
+	return b[off : off+size : off+size]
+}
+
+func isODirectMemAligned(b []byte) bool {
+	return len(b) == 0 || uintptr(unsafe.Pointer(&b[0]))%odirectAlign == 0
+}
 
 func (tmp *tmpfile) Write(b []byte) (int, error) {
 	if int64(len(b)) > tmp.size {
@@ -33,7 +52,9 @@ func (tmp *tmpfile) Write(b []byte) (int, error) {
 	}
 
 	if tmp.useODirect && !isODirectLenAligned(len(b)) {
-		if err := tmp.switchToBufferedAtCurrentOffset(fmt.Sprintf("unaligned write length: len=%d len%%512=%d", len(b), len(b)%odirectMinWriteAlign)); err != nil {
+		reason := fmt.Sprintf("unaligned write length: len=%d len%%%d=%d", len(b), odirectAlign, len(b)%odirectAlign)
+		debuglogger.Logf("O_DIRECT tmpfile falling back to buffered I/O (%s/%s): %s", tmp.bucket, tmp.objname, reason)
+		if err := tmp.switchToBufferedAtCurrentOffset(reason); err != nil {
 			return 0, err
 		}
 	}
@@ -51,6 +72,73 @@ func (tmp *tmpfile) Write(b []byte) (int, error) {
 	return n, err
 }
 
+// copyFrom copies r into tmp. With O_DIRECT it fills buf completely before
+// each write so that only the final tail of the object is unaligned.
+func (tmp *tmpfile) copyFrom(r io.Reader, buf []byte) (int64, error) {
+	if !tmp.useODirect || len(buf) < odirectAlign {
+		return io.CopyBuffer(tmp, r, buf)
+	}
+	if !isODirectMemAligned(buf) {
+		buf = alignedBuffer(len(buf))
+	}
+	buf = buf[:len(buf)-len(buf)%odirectAlign]
+
+	var written int64
+	for {
+		// Not io.ReadFull: it drops errors returned alongside a full buffer,
+		// which is how HashReader reports checksum mismatches.
+		n := 0
+		var rerr error
+		for n < len(buf) && rerr == nil {
+			var nn int
+			nn, rerr = r.Read(buf[n:])
+			n += nn
+		}
+
+		if n > 0 {
+			w, werr := tmp.writeAlignedThenTail(buf[:n])
+			written += int64(w)
+			if werr != nil {
+				return written, werr
+			}
+		}
+
+		if rerr == io.EOF {
+			return written, nil
+		}
+		if rerr != nil {
+			return written, rerr
+		}
+	}
+}
+
+// writeAlignedThenTail writes the block-aligned prefix of b with O_DIRECT and
+// any unaligned remainder buffered, without treating the tail as a fallback.
+func (tmp *tmpfile) writeAlignedThenTail(b []byte) (int, error) {
+	if !tmp.useODirect || isODirectLenAligned(len(b)) {
+		return tmp.Write(b)
+	}
+
+	aligned := len(b) - len(b)%odirectAlign
+	n := 0
+	if aligned > 0 {
+		var err error
+		n, err = tmp.Write(b[:aligned])
+		if err != nil {
+			return n, err
+		}
+	}
+
+	if tmp.useODirect {
+		if err := tmp.switchToBufferedAtCurrentOffset("final unaligned tail"); err != nil {
+			return n, err
+		}
+	}
+
+	m, err := tmp.Write(b[aligned:])
+	return n + m, err
+}
+
 func (tmp *tmpfile) switchToBufferedAtCurrentOffset(reason string) error {
 	offset, seekErr := tmp.f.Seek(0, io.SeekCurrent)
 	if seekErr != nil {
@@ -83,7 +171,7 @@ func (tmp *tmpfile) switchToBufferedAtCurrentOffset(reason string) error {
 }
 
 func isODirectLenAligned(n int) bool {
-	return n%odirectMinWriteAlign == 0
+	return n%odirectAlign == 0
 }
 
 func isODirectRuntimeFallbackErr(err error) bool {
diff --git a/backend/posix/otmpfile_common_test.go b/backend/posix/otmpfile_common_test.go
new file mode 100644
index 00000000..650e1f04
--- /dev/null
+++ b/backend/posix/otmpfile_common_test.go
@@ -0,0 +1,152 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package posix
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// shortReader returns at most max bytes per Read, and errAtEOF with the final data.
+type shortReader struct {
+	r        io.Reader
+	max      int
+	errAtEOF error
+}
+
+func (s *shortReader) Read(p []byte) (int, error) {
+	if len(p) > s.max {
+		p = p[:s.max]
+	}
+	n, err := s.r.Read(p)
+	if err == nil && s.errAtEOF != nil {
+		if br, ok := s.r.(*bytes.Reader); ok && br.Len() == 0 {
+			return n, s.errAtEOF
+		}
+	}
+	return n, err
+}
+
+// newTestODirectTmpfile builds a tmpfile over a regular file with useODirect
+// set, so alignment decisions can be observed without O_DIRECT support.
+func newTestODirectTmpfile(t *testing.T, size int64) *tmpfile {
+	t.Helper()
+	f, err := os.Create(filepath.Join(t.TempDir(), "obj"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { f.Close() })
+	return &tmpfile{f: f, useODirect: true, size: size}
+}
+
+func TestTmpfileCopyFromShortReads(t *testing.T) {
+	tests := []struct {
+		name         string
+		size         int
+		wantODirect  bool
+		bufSize      int
+		maxReadBytes int
+	}{
+		{"aligned size stays O_DIRECT", 4 * 1024 * 1024, true, 1024 * 1024, 7777},
+		{"unaligned tail switches at end", 4*1024*1024 + 123, false, 1024 * 1024, 7777},
+		{"512 aligned tail is not 4k aligned", 3*4096 + 512, false, 1024 * 1024, 7777},
+		{"small unaligned object", 100, false, 1024 * 1024, 13},
+		{"unaligned buffer size", 64 * 1024, true, 10000, 999},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			data := make([]byte, tc.size)
+			rand.New(rand.NewSource(1)).Read(data)
+
+			tmp := newTestODirectTmpfile(t, int64(tc.size))
+			rdr := &shortReader{r: bytes.NewReader(data), max: tc.maxReadBytes}
+
+			n, err := tmp.copyFrom(rdr, make([]byte, tc.bufSize))
+			if err != nil {
+				t.Fatalf("copyFrom: %v", err)
+			}
+			if n != int64(tc.size) {
+				t.Fatalf("copied %d, want %d", n, tc.size)
+			}
+			if tmp.useODirect != tc.wantODirect {
+				t.Fatalf("useODirect=%v, want %v", tmp.useODirect, tc.wantODirect)
+			}
+
+			got, err := os.ReadFile(tmp.f.Name())
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(got, data) {
+				t.Fatal("file content mismatch")
+			}
+		})
+	}
+}
+
+func TestAlignedBuffer(t *testing.T) {
+	for _, size := range []int{1, 512, 4096, 10000, 1024 * 1024} {
+		b := alignedBuffer(size)
+		if len(b) != size || cap(b) != size {
+			t.Fatalf("size %d: len=%d cap=%d", size, len(b), cap(b))
+		}
+		if !isODirectMemAligned(b) {
+			t.Fatalf("size %d: buffer not %d-byte aligned", size, odirectAlign)
+		}
+	}
+}
+
+func TestTmpfileCopyFromUnalignedMemory(t *testing.T) {
+	data := make([]byte, 64*1024)
+	rand.New(rand.NewSource(2)).Read(data)
+
+	tmp := newTestODirectTmpfile(t, int64(len(data)))
+	buf := alignedBuffer(32*1024 + 1)[1:]
+	if isODirectMemAligned(buf) {
+		t.Fatal("test buffer unexpectedly aligned")
+	}
+
+	if _, err := tmp.copyFrom(&shortReader{r: bytes.NewReader(data), max: 1000}, buf); err != nil {
+		t.Fatalf("copyFrom: %v", err)
+	}
+	if !tmp.useODirect {
+		t.Fatal("fell back to buffered I/O")
+	}
+	got, err := os.ReadFile(tmp.f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(got, data) {
+		t.Fatal("file content mismatch")
+	}
+}
+
+func TestTmpfileCopyFromPropagatesErrorWithFinalData(t *testing.T) {
+	errBad := errors.New("bad digest")
+	data := make([]byte, 1024*1024)
+
+	tmp := newTestODirectTmpfile(t, int64(len(data)))
+	rdr := &shortReader{r: bytes.NewReader(data), max: 4096, errAtEOF: errBad}
+
+	_, err := tmp.copyFrom(rdr, make([]byte, len(data)))
+	if !errors.Is(err, errBad) {
+		t.Fatalf("got err %v, want %v", err, errBad)
+	}
+}
diff --git a/backend/posix/posix.go b/backend/posix/posix.go
index 4c36ca48..b30a63a6 100644
--- a/backend/posix/posix.go
+++ b/backend/posix/posix.go
@@ -422,7 +422,7 @@ func New(rootdir string, ms meta.MetadataStorer, opts PosixOpts) (*Posix, error)
 		defaultEtag:          opts.DefaultEtag,
 		ioBufferSize:         ioBufferSize,
 		ioBufferPool: sync.Pool{New: func() any {
-			b := make([]byte, ioBufferSize)
+			b := alignedBuffer(ioBufferSize)
 			return &b
 		}},
 		dataIntegrityEtag: opts.DataIntegrityEtag,
@@ -484,7 +484,7 @@ func ioBufferSizeOrDefault(n int) int {
 func (p *Posix) getIOBuffer() []byte {
 	bp, ok := p.ioBufferPool.Get().(*[]byte)
 	if !ok || bp == nil || cap(*bp) < p.ioBufferSize {
-		return make([]byte, p.ioBufferSize)
+		return alignedBuffer(p.ioBufferSize)
 	}
 	return (*bp)[:p.ioBufferSize]
 }
@@ -3921,7 +3921,7 @@ func (p *Posix) UploadPartWithPostFunc(ctx context.Context, input *s3.UploadPart
 	buf := p.getIOBuffer()
 	defer p.putIOBuffer(buf)
 
-	_, err = io.CopyBuffer(f, tr, buf)
+	_, err = f.copyFrom(tr, buf)
 	if err != nil {
 		if errors.Is(err, syscall.EDQUOT) {
 			drainBody(tr)
@@ -4805,7 +4805,7 @@ func (p *Posix) PutObjectWithPostFunc(ctx context.Context, po s3response.PutObje
 	buf := p.getIOBuffer()
 	defer p.putIOBuffer(buf)
 
-	_, err = io.CopyBuffer(f, rdr, buf)
+	_, err = f.copyFrom(rdr, buf)
 	if err != nil {
 		if errors.Is(err, syscall.EDQUOT) {
 			drainBody(rdr)
@@ -6038,8 +6038,7 @@ func (p *Posix) GetObject(ctx context.Context, input *s3.GetObjectInput) (*s3.Ge
 		}
 	}
 
-	// Full-object responses can keep the underlying *os.File for sendfile.
-	// Linux range reads on O_DIRECT may need runtime fallback to buffered I/O.
+	// Buffered full-object responses keep the underlying *os.File for sendfile.
 	body, err := buildGetObjectBody(f, objPath, startOffset, length, objSize, p.enableODirect, p.ioBufferSize)
 	if err != nil {
 		return nil, fmt.Errorf("build get object body: %w", err)