[Qemu-devel] [PATCH] Improve qemu-nbd performance by 4400 %

[Laurent Vivier]

This patch allows to reduce the boot time from an NBD server from 225 seconds to
5 seconds (time between the "boot cd:0" and the kernel init) for the
following command lines:

./qemu-nbd -t ../ISO/debian-500-powerpc-netinst.iso
and
./ppc-softmmu/qemu-system-ppc -cdrom nbd:localhost:1024

Signed-off-by: Laurent Vivier <address@hidden>
---
 nbd.c |   20 +++++++++++++++-----
 1 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/nbd.c b/nbd.c
index 011b50f..5d7c758 100644
--- a/nbd.c
+++ b/nbd.c
@@ -655,7 +655,7 @@ int nbd_trip(BlockDriverState *bs, int csock, off_t size, 
uint64_t dev_offset,
        if (nbd_receive_request(csock, &request) == -1)
                return -1;
 
-       if (request.len > data_size) {
+       if (request.len + sizeof(struct nbd_reply) > data_size) {
                LOG("len (%u) is larger than max len (%u)",
                    request.len, data_size);
                errno = EINVAL;
@@ -687,7 +687,8 @@ int nbd_trip(BlockDriverState *bs, int csock, off_t size, 
uint64_t dev_offset,
        case NBD_CMD_READ:
                TRACE("Request type is READ");
 
-               if (bdrv_read(bs, (request.from + dev_offset) / 512, data,
+               if (bdrv_read(bs, (request.from + dev_offset) / 512,
+                             data + sizeof(struct nbd_reply),
                              request.len / 512) == -1) {
                        LOG("reading from file failed");
                        errno = EINVAL;
@@ -697,12 +698,21 @@ int nbd_trip(BlockDriverState *bs, int csock, off_t size, 
uint64_t dev_offset,
 
                TRACE("Read %u byte(s)", request.len);
 
-               if (nbd_send_reply(csock, &reply) == -1)
-                       return -1;
+               /* Reply
+                  [ 0 ..  3]    magic   (NBD_REPLY_MAGIC)
+                  [ 4 ..  7]    error   (0 == no error)
+                  [ 7 .. 15]    handle
+                */
+
+               cpu_to_be32w((uint32_t*)data, NBD_REPLY_MAGIC);
+               cpu_to_be32w((uint32_t*)(data + 4), reply.error);
+               cpu_to_be64w((uint64_t*)(data + 8), reply.handle);
 
                TRACE("Sending data to client");
 
-               if (write_sync(csock, data, request.len) != request.len) {
+               if (write_sync(csock, data,
+                              request.len + sizeof(struct nbd_reply)) !=
+                              request.len + sizeof(struct nbd_reply)) {
                        LOG("writing to socket failed");
                        errno = EINVAL;
                        return -1;
-- 
1.7.0.4