Index: sys/fs/fuse/fuse_vfsops.c
===================================================================
--- sys/fs/fuse/fuse_vfsops.c
+++ sys/fs/fuse/fuse_vfsops.c
@@ -433,7 +433,12 @@
 	 */
 	mp->mnt_kern_flag |= MNTK_NULL_NOCACHE;
 	MNT_IUNLOCK(mp);
-	/* We need this here as this slot is used by getnewvnode() */
+	/*
+	 * mnt_stat.f_iosize is used by the buffer cache.  We choose a sensible
+	 * value ourselves, rather than trusting what the FUSE server reports.
+	 * But this is distinct from the f_iosize that we will report via
+	 * statfs(2).
+	 */
 	mp->mnt_stat.f_iosize = maxbcachebuf;
 	if (subtype) {
 		strlcat(mp->mnt_stat.f_fstypename, ".", MFSNAMELEN);
@@ -682,6 +687,10 @@
 	sbp->f_ffree = fsfo->st.ffree;	/* cast from uint64_t to int64_t */
 	sbp->f_namemax = fsfo->st.namelen;
 	sbp->f_bsize = fsfo->st.frsize;	/* cast from uint32_t to uint64_t */
+	if (fsess_opt_datacache(mp))
+		sbp->f_iosize = MAX(fsfo->st.bsize, mp->mnt_stat.f_iosize);
+	else
+		sbp->f_iosize = fsfo->st.bsize;
 
 	fdisp_destroy(&fdi);
 	return 0;
Index: tests/sys/fs/fusefs/statfs.cc
===================================================================
--- tests/sys/fs/fusefs/statfs.cc
+++ tests/sys/fs/fusefs/statfs.cc
@@ -33,6 +33,7 @@
 extern "C" {
 #include <sys/param.h>
 #include <sys/mount.h>
+#include <sys/sysctl.h>
 #include <semaphore.h>
 }
 
@@ -43,6 +44,15 @@
 
 class Statfs: public FuseTest {};
 
+class Statfs_cached: public Statfs {
+public:
+virtual void SetUp() {
+	m_init_flags |= FUSE_WRITEBACK_CACHE;
+	Statfs::SetUp();
+}
+};
+
+
 TEST_F(Statfs, eio)
 {
 	struct statfs statbuf;
@@ -150,24 +160,57 @@
 		out.body.statfs.st.ffree = 6;
 		out.body.statfs.st.namelen = 128;
 		out.body.statfs.st.frsize = 1024;
+		out.body.statfs.st.bsize = 131072;
 	})));
 
 	ASSERT_NE(nullptr, getcwd(mp, PATH_MAX)) << strerror(errno);
 	strlcat(mp, "/mountpoint", PATH_MAX);
 	ASSERT_EQ(0, statfs("mountpoint", &statbuf)) << strerror(errno);
 	EXPECT_EQ(1024ul, statbuf.f_bsize);
-	/* 
-	 * fuse(4) ignores the filesystem's reported optimal transfer size, and
-	 * chooses a size that works well with the rest of the system instead
-	 */
 	EXPECT_EQ(1000ul, statbuf.f_blocks);
 	EXPECT_EQ(100ul, statbuf.f_bfree);
 	EXPECT_EQ(200l, statbuf.f_bavail);
 	EXPECT_EQ(5ul, statbuf.f_files);
 	EXPECT_EQ(6l, statbuf.f_ffree);
+	EXPECT_EQ(131072ul, statbuf.f_iosize);
 	EXPECT_EQ(128u, statbuf.f_namemax);
 	EXPECT_EQ(getuid(), statbuf.f_owner);
 	EXPECT_EQ(0, strcmp("fusefs", statbuf.f_fstypename));
 	EXPECT_EQ(0, strcmp("/dev/fuse", statbuf.f_mntfromname));
 	EXPECT_EQ(0, strcmp(mp, statbuf.f_mntonname));
 }
+
+/*
+ * When not using direct I/O fusefs will choose a minimum f_iosize suitable for
+ * the buffer cache.
+ */
+TEST_F(Statfs_cached, ok)
+{
+	struct statfs statbuf;
+	int maxbcachebuf;
+	const char *node = "vfs.maxbcachebuf";
+	size_t size = sizeof(maxbcachebuf);
+
+	ASSERT_EQ(0, sysctlbyname(node, &maxbcachebuf, &size, NULL, 0))
+		<< strerror(errno);
+
+	EXPECT_CALL(*m_mock, process(
+		ResultOf([](auto in) {
+			return (in.header.opcode == FUSE_STATFS);
+		}, Eq(true)),
+		_)
+	).WillOnce(Invoke(ReturnImmediate([=](auto in __unused, auto& out) {
+		SET_OUT_HEADER_LEN(out, statfs);
+		out.body.statfs.st.blocks = 1000;
+		out.body.statfs.st.bfree = 100;
+		out.body.statfs.st.bavail = 200;
+		out.body.statfs.st.files = 5;
+		out.body.statfs.st.ffree = 6;
+		out.body.statfs.st.namelen = 128;
+		out.body.statfs.st.frsize = 1024;
+		out.body.statfs.st.bsize = 8192;
+	})));
+
+	ASSERT_EQ(0, statfs("mountpoint", &statbuf)) << strerror(errno);
+	EXPECT_EQ((uint64_t)maxbcachebuf, statbuf.f_iosize);
+}