block/iscsi: optimize WRITE10/16 if cache.writeback is not set

SCSI allowes to tell the target to not return from a write command
if the date is not written to the disk. Use this so called FUA
bit if it is supported to optimize WRITE commands if writeback is
not allowed.

In this case qemu always issues a WRITE followed by a FLUSH. This
is 2 round trip times. If we set the FUA bit we can ignore the
following FLUSH.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Message-id: 1429193313-4263-6-git-send-email-pl@kamp.de
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/iscsi.c

@@ -68,6 +68,7 @@ typedef struct IscsiLun {
     bool lbprz;
     bool dpofua;
     bool has_write_same;
+    bool force_next_flush;
 } IscsiLun;
 
 typedef struct IscsiTask {
@@ -80,6 +81,7 @@ typedef struct IscsiTask {
     QEMUBH *bh;
     IscsiLun *iscsilun;
     QEMUTimer retry_timer;
+    bool force_next_flush;
 } IscsiTask;
 
 typedef struct IscsiAIOCB {
@@ -200,6 +202,8 @@ iscsi_co_generic_cb(struct iscsi_context *iscsi, int status,
             }
         }
         error_report("iSCSI Failure: %s", iscsi_get_error(iscsi));
+    } else {
+        iTask->iscsilun->force_next_flush |= iTask->force_next_flush;
     }
 
 out:
@@ -370,6 +374,7 @@ static int coroutine_fn iscsi_co_writev(BlockDriverState *bs,
     struct IscsiTask iTask;
     uint64_t lba;
     uint32_t num_sectors;
+    int fua;
 
     if (!is_request_lun_aligned(sector_num, nb_sectors, iscsilun)) {
         return -EINVAL;
@@ -385,15 +390,17 @@ static int coroutine_fn iscsi_co_writev(BlockDriverState *bs,
     num_sectors = sector_qemu2lun(nb_sectors, iscsilun);
     iscsi_co_init_iscsitask(iscsilun, &iTask);
 retry:
+    fua = iscsilun->dpofua && !bs->enable_write_cache;
+    iTask.force_next_flush = !fua;
     if (iscsilun->use_16_for_rw) {
         iTask.task = iscsi_write16_task(iscsilun->iscsi, iscsilun->lun, lba,
                                         NULL, num_sectors * iscsilun->block_size,
-                                        iscsilun->block_size, 0, 0, 0, 0, 0,
+                                        iscsilun->block_size, 0, 0, fua, 0, 0,
                                         iscsi_co_generic_cb, &iTask);
     } else {
         iTask.task = iscsi_write10_task(iscsilun->iscsi, iscsilun->lun, lba,
                                         NULL, num_sectors * iscsilun->block_size,
-                                        iscsilun->block_size, 0, 0, 0, 0, 0,
+                                        iscsilun->block_size, 0, 0, fua, 0, 0,
                                         iscsi_co_generic_cb, &iTask);
     }
     if (iTask.task == NULL) {
@@ -621,8 +628,12 @@ static int coroutine_fn iscsi_co_flush(BlockDriverState *bs)
         return 0;
     }
 
-    iscsi_co_init_iscsitask(iscsilun, &iTask);
+    if (!iscsilun->force_next_flush) {
+        return 0;
+    }
+    iscsilun->force_next_flush = false;
 
+    iscsi_co_init_iscsitask(iscsilun, &iTask);
 retry:
     if (iscsi_synchronizecache10_task(iscsilun->iscsi, iscsilun->lun, 0, 0, 0,
                                       0, iscsi_co_generic_cb, &iTask) == NULL) {
@@ -918,6 +929,7 @@ coroutine_fn iscsi_co_write_zeroes(BlockDriverState *bs, int64_t sector_num,
     }
 
     iscsi_co_init_iscsitask(iscsilun, &iTask);
+    iTask.force_next_flush = true;
 retry:
     if (use_16_for_ws) {
         iTask.task = iscsi_writesame16_task(iscsilun->iscsi, iscsilun->lun, lba,