block/iscsi: clarify the meaning of ISCSI_CHECKALLOC_THRES
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/block/iscsi.c b/block/iscsi.c
index 6b8a0a3..e8d26bb 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -95,7 +95,15 @@
#define NOP_INTERVAL 5000
#define MAX_NOP_FAILURES 3
#define ISCSI_CMD_RETRIES 5
-#define ISCSI_CHECKALLOC_THRES 63
+
+/* this threshhold is a trade-off knob to choose between
+ * the potential additional overhead of an extra GET_LBA_STATUS request
+ * vs. unnecessarily reading a lot of zero sectors over the wire.
+ * If a read request is greater or equal than ISCSI_CHECKALLOC_THRES
+ * sectors we check the allocation status of the area covered by the
+ * request first if the allocationmap indicates that the area might be
+ * unallocated. */
+#define ISCSI_CHECKALLOC_THRES 64
static void
iscsi_bh_cb(void *p)
@@ -505,7 +513,7 @@
}
#if defined(LIBISCSI_FEATURE_IOVECTOR)
- if (iscsilun->lbprz && nb_sectors > ISCSI_CHECKALLOC_THRES &&
+ if (iscsilun->lbprz && nb_sectors >= ISCSI_CHECKALLOC_THRES &&
!iscsi_allocationmap_is_allocated(iscsilun, sector_num, nb_sectors)) {
int64_t ret;
int pnum;
