diff --git a/src/fio_cluster.cpp b/src/fio_cluster.cpp
index 78419eaa..edc82822 100644
--- a/src/fio_cluster.cpp
+++ b/src/fio_cluster.cpp
@@ -186,10 +186,10 @@ static struct fio_option options[] = {
     },
 };
 
-static void watch_callback(long retval, void *opaque)
+static void watch_callback(void *opaque, long watch)
 {
     struct sec_data *bsd = (struct sec_data*)opaque;
-    bsd->watch = (void*)retval;
+    bsd->watch = (void*)watch;
 }
 
 static int sec_setup(struct thread_data *td)
@@ -274,7 +274,7 @@ static int sec_init(struct thread_data *td)
     return 0;
 }
 
-static void io_callback(long retval, void *opaque)
+static void io_callback(void *opaque, long retval)
 {
     struct io_u *io = (struct io_u*)opaque;
     io->error = retval < 0 ? -retval : 0;
@@ -288,6 +288,11 @@ static void io_callback(long retval, void *opaque)
     }
 }
 
+static void read_callback(void *opaque, long retval, uint64_t version)
+{
+    io_callback(opaque, retval);
+}
+
 /* Begin read or write request. */
 static enum fio_q_status sec_queue(struct thread_data *td, struct io_u *io)
 {
@@ -310,7 +315,7 @@ static enum fio_q_status sec_queue(struct thread_data *td, struct io_u *io)
     {
     case DDIR_READ:
         iov = { .iov_base = io->xfer_buf, .iov_len = io->xfer_buflen };
-        vitastor_c_read(bsd->cli, inode, io->offset, io->xfer_buflen, &iov, 1, io_callback, io);
+        vitastor_c_read(bsd->cli, inode, io->offset, io->xfer_buflen, &iov, 1, read_callback, io);
         bsd->last_sync = false;
         break;
     case DDIR_WRITE:
@@ -320,7 +325,7 @@ static enum fio_q_status sec_queue(struct thread_data *td, struct io_u *io)
             return FIO_Q_COMPLETED;
         }
         iov = { .iov_base = io->xfer_buf, .iov_len = io->xfer_buflen };
-        vitastor_c_write(bsd->cli, inode, io->offset, io->xfer_buflen, &iov, 1, io_callback, io);
+        vitastor_c_write(bsd->cli, inode, io->offset, io->xfer_buflen, 0, &iov, 1, io_callback, io);
         bsd->last_sync = false;
         break;
     case DDIR_SYNC:
diff --git a/src/qemu_driver.c b/src/qemu_driver.c
index 4e647f26..5c282da1 100644
--- a/src/qemu_driver.c
+++ b/src/qemu_driver.c
@@ -66,7 +66,8 @@ typedef struct VitastorRPC
 } VitastorRPC;
 
 static void vitastor_co_init_task(BlockDriverState *bs, VitastorRPC *task);
-static void vitastor_co_generic_bh_cb(long retval, void *opaque);
+static void vitastor_co_generic_bh_cb(void *opaque, long retval);
+static void vitastor_co_read_cb(void *opaque, long retval, uint64_t version);
 static void vitastor_close(BlockDriverState *bs);
 
 static char *qemu_rbd_next_tok(char *src, char delim, char **p)
@@ -391,7 +392,7 @@ static void vitastor_co_init_task(BlockDriverState *bs, VitastorRPC *task)
     };
 }
 
-static void vitastor_co_generic_bh_cb(long retval, void *opaque)
+static void vitastor_co_generic_bh_cb(void *opaque, long retval)
 {
     VitastorRPC *task = opaque;
     task->ret = retval;
@@ -407,6 +408,11 @@ static void vitastor_co_generic_bh_cb(long retval, void *opaque)
     }
 }
 
+static void vitastor_co_read_cb(void *opaque, long retval, uint64_t version)
+{
+    vitastor_co_generic_bh_cb(opaque, retval);
+}
+
 static int coroutine_fn vitastor_co_preadv(BlockDriverState *bs, uint64_t offset, uint64_t bytes, QEMUIOVector *iov, int flags)
 {
     VitastorClient *client = bs->opaque;
@@ -416,7 +422,7 @@ static int coroutine_fn vitastor_co_preadv(BlockDriverState *bs, uint64_t offset
 
     uint64_t inode = client->watch ? vitastor_c_inode_get_num(client->watch) : client->inode;
     qemu_mutex_lock(&client->mutex);
-    vitastor_c_read(client->proxy, inode, offset, bytes, iov->iov, iov->niov, vitastor_co_generic_bh_cb, &task);
+    vitastor_c_read(client->proxy, inode, offset, bytes, iov->iov, iov->niov, vitastor_co_read_cb, &task);
     qemu_mutex_unlock(&client->mutex);
 
     while (!task.complete)
@@ -436,7 +442,7 @@ static int coroutine_fn vitastor_co_pwritev(BlockDriverState *bs, uint64_t offse
 
     uint64_t inode = client->watch ? vitastor_c_inode_get_num(client->watch) : client->inode;
     qemu_mutex_lock(&client->mutex);
-    vitastor_c_write(client->proxy, inode, offset, bytes, iov->iov, iov->niov, vitastor_co_generic_bh_cb, &task);
+    vitastor_c_write(client->proxy, inode, offset, bytes, 0, iov->iov, iov->niov, vitastor_co_generic_bh_cb, &task);
     qemu_mutex_unlock(&client->mutex);
 
     while (!task.complete)
diff --git a/src/vitastor_c.cpp b/src/vitastor_c.cpp
index 9f6cfa6d..9300888e 100644
--- a/src/vitastor_c.cpp
+++ b/src/vitastor_c.cpp
@@ -166,11 +166,11 @@ void vitastor_c_uring_wait_events(vitastor_c *client)
     client->ringloop->wait();
 }
 
-static inline void vitastor_c_rw(bool write, vitastor_c *p, uint64_t inode, uint64_t offset, uint64_t len,
-    struct iovec *iov, int iovcnt, VitastorIOHandler cb, void *opaque)
+void vitastor_c_read(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len,
+    struct iovec *iov, int iovcnt, VitastorReadHandler cb, void *opaque)
 {
     cluster_op_t *op = new cluster_op_t;
-    op->opcode = write ? OSD_OP_WRITE : OSD_OP_READ;
+    op->opcode = OSD_OP_READ;
     op->inode = inode;
     op->offset = offset;
     op->len = len;
@@ -180,22 +180,31 @@ static inline void vitastor_c_rw(bool write, vitastor_c *p, uint64_t inode, uint
     }
     op->callback = [cb, opaque](cluster_op_t *op)
     {
-        cb(op->retval, opaque);
+        cb(opaque, op->retval, op->version);
         delete op;
     };
-    p->cli->execute(op);
+    client->cli->execute(op);
 }
 
-void vitastor_c_read(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len,
+void vitastor_c_write(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len, uint64_t check_version,
     struct iovec *iov, int iovcnt, VitastorIOHandler cb, void *opaque)
 {
-    vitastor_c_rw(0, client, inode, offset, len, iov, iovcnt, cb, opaque);
-}
-
-void vitastor_c_write(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len,
-    struct iovec *iov, int iovcnt, VitastorIOHandler cb, void *opaque)
-{
-    vitastor_c_rw(1, client, inode, offset, len, iov, iovcnt, cb, opaque);
+    cluster_op_t *op = new cluster_op_t;
+    op->opcode = OSD_OP_WRITE;
+    op->inode = inode;
+    op->offset = offset;
+    op->len = len;
+    op->version = check_version;
+    for (int i = 0; i < iovcnt; i++)
+    {
+        op->iov.push_back(iov[i].iov_base, iov[i].iov_len);
+    }
+    op->callback = [cb, opaque](cluster_op_t *op)
+    {
+        cb(opaque, op->retval);
+        delete op;
+    };
+    client->cli->execute(op);
 }
 
 void vitastor_c_sync(vitastor_c *client, VitastorIOHandler cb, void *opaque)
@@ -204,7 +213,7 @@ void vitastor_c_sync(vitastor_c *client, VitastorIOHandler cb, void *opaque)
     op->opcode = OSD_OP_SYNC;
     op->callback = [cb, opaque](cluster_op_t *op)
     {
-        cb(op->retval, opaque);
+        cb(opaque, op->retval);
         delete op;
     };
     client->cli->execute(op);
@@ -215,7 +224,7 @@ void vitastor_c_watch_inode(vitastor_c *client, char *image, VitastorIOHandler c
     client->cli->on_ready([=]()
     {
         auto watch = client->cli->st_cli.watch_inode(std::string(image));
-        cb((long)watch, opaque);
+        cb(opaque, (long)watch);
     });
 }
 
diff --git a/src/vitastor_c.h b/src/vitastor_c.h
index 7c816dff..f964c642 100644
--- a/src/vitastor_c.h
+++ b/src/vitastor_c.h
@@ -19,7 +19,8 @@ extern "C" {
 struct vitastor_c;
 typedef struct vitastor_c vitastor_c;
 
-typedef void VitastorIOHandler(long retval, void *opaque);
+typedef void VitastorReadHandler(void *opaque, long retval, uint64_t version);
+typedef void VitastorIOHandler(void *opaque, long retval);
 
 // QEMU
 typedef void IOHandler(void *opaque);
@@ -37,8 +38,8 @@ void vitastor_c_uring_wait_ready(vitastor_c *client);
 void vitastor_c_uring_handle_events(vitastor_c *client);
 void vitastor_c_uring_wait_events(vitastor_c *client);
 void vitastor_c_read(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len,
-    struct iovec *iov, int iovcnt, VitastorIOHandler cb, void *opaque);
-void vitastor_c_write(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len,
+    struct iovec *iov, int iovcnt, VitastorReadHandler cb, void *opaque);
+void vitastor_c_write(vitastor_c *client, uint64_t inode, uint64_t offset, uint64_t len, uint64_t check_version,
     struct iovec *iov, int iovcnt, VitastorIOHandler cb, void *opaque);
 void vitastor_c_sync(vitastor_c *client, VitastorIOHandler cb, void *opaque);
 void vitastor_c_watch_inode(vitastor_c *client, char *image, VitastorIOHandler cb, void *opaque);