Max out the size of reads and writes from the kernel.

Fixes googlecloudplatform/gcsfuse#27.

fuseops/convert.go

@@ -36,7 +36,10 @@ func Convert(r bazilfuse.Request, logger *log.Logger) (o Op) {
 
 	switch typed := r.(type) {
 	case *bazilfuse.InitRequest:
-		to := &InitOp{}
+		to := &InitOp{
+			maxReadahead: typed.MaxReadahead,
+		}
+
 		o = to
 		co = &to.commonOp
 

fuseops/ops.go

@@ -48,6 +48,8 @@ type Op interface {
 // mount to succeed.
 type InitOp struct {
 	commonOp
+
+	maxReadahead uint32
 }
 
 func (o *InitOp) Respond(err error) {
@@ -56,10 +58,50 @@ func (o *InitOp) Respond(err error) {
 		return
 	}
 
-	resp := &bazilfuse.InitResponse{}
+	resp := bazilfuse.InitResponse{}
 
+	// Ask the Linux kernel for larger write requests.
+	//
+	// As of 2015-03-26, the behavior in the kernel is:
+	//
+	//  *  (http://goo.gl/jMKHMZ, http://goo.gl/XTF4ZH) Cap the max write size at
+	//     the maximum of 4096 and init_response->max_write.
+	//
+	//  *  (http://goo.gl/gEIvHZ) If FUSE_BIG_WRITES isn't set, don't return more
+	//     than one page.
+	//
+	//  *  (http://goo.gl/4RLhxZ, http://goo.gl/hi0Cm2) Never write more than
+	//     FUSE_MAX_PAGES_PER_REQ pages (128 KiB on x86).
+	//
+	// 4 KiB is crazy small. Ask for significantly more, and take what the kernel
+	// will give us.
+	const maxWrite = 1 << 21
+	resp.Flags |= bazilfuse.InitBigWrites
+	resp.MaxWrite = maxWrite
+
+	// Ask the Linux kernel for larger read requests.
+	//
+	// As of 2015-03-26, the behavior in the kernel is:
+	//
+	//  *  (http://goo.gl/bQ1f1i, http://goo.gl/HwBrR6) Set the local variable
+	//     ra_pages to be init_response->max_readahead divided by the page size.
+	//
+	//  *  (http://goo.gl/gcIsSh, http://goo.gl/LKV2vA) Set
+	//     backing_dev_info::ra_pages to the min of that value and what was sent
+	//     in the request's max_readahead field.
+	//
+	//  *  (http://goo.gl/u2SqzH) Use backing_dev_info::ra_pages when deciding
+	//     how much to read ahead.
+	//
+	//  *  (http://goo.gl/JnhbdL) Don't read ahead at all if that field is zero.
+	//
+	// Reading a page at a time is a drag. Ask for as much as the kernel is
+	// willing to give us.
+	resp.MaxReadahead = o.maxReadahead
+
+	// Respond.
 	o.commonOp.logger.Printf("Responding: %v", &resp)
-	o.r.(*bazilfuse.InitRequest).Respond(resp)
+	o.r.(*bazilfuse.InitRequest).Respond(&resp)
 }
 
 ////////////////////////////////////////////////////////////////////////

samples/memfs/memfs_test.go

@@ -15,6 +15,7 @@
 package memfs_test
 
 import (
+	"bytes"
 	"io"
 	"io/ioutil"
 	"os"
@@ -825,6 +826,27 @@ func (t *MemFSTest) WriteAtDoesntChangeOffset_AppendMode() {
 	ExpectEq(4, offset)
 }
 
+func (t *MemFSTest) LargeFile() {
+	var err error
+
+	// Create a file.
+	f, err := os.Create(path.Join(t.Dir, "foo"))
+	t.ToClose = append(t.ToClose, f)
+	AssertEq(nil, err)
+
+	// Copy in large contents.
+	const size = 1 << 24
+	contents := bytes.Repeat([]byte{0x20}, size)
+
+	_, err = io.Copy(f, bytes.NewReader(contents))
+	AssertEq(nil, err)
+
+	// Read the full contents of the file.
+	contents, err = ioutil.ReadFile(f.Name())
+	AssertEq(nil, err)
+	ExpectEq(size, len(contents))
+}
+
 func (t *MemFSTest) AppendMode() {
 	var err error
 	var n int