[PATCH v4 1/8] zbd: fix zone selection of random writes

In zonemode=zbd, random write workloads targeting zoned block devices
with max write zones limits such as max_open_zones can not do write
operations to randomly chosen offset because of the zoned block device
constraint of writing at write pointers. To adjust the offsets to valid
positions, fio calls the function zbd_convert_to_write_zone(). This
function checks the current write target zones as the next offset
candidates but may fail depending on the conditions of those zones.
In such cases, the function waits for zone condition changes before
retrying.

However, the retry logic begins with the zone where the previous attempt
ended, and selects the zones that were previously write target.
Consequently, the same zones are repeatedly chosen for writing,
resulting in writes concentrating on certain zones despite the workload
specifying random write.

To ensure proper zone selection for random writes, modify
zbd_convert_to_write_zone() to retry the zone selection based on the
original offset provided to the function. The local variable 'zb' keeps
the reference to the zone corresponding to the original offset. Use 'zb'
at the retry attempt start.

Reviewed-by: Damien Le Moal <[email protected]>
Signed-off-by: Shin'ichiro Kawasaki <[email protected]>
---
 zbd.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/zbd.c b/zbd.c
index 08c537d7..cf825601 100644
--- a/zbd.c
+++ b/zbd.c
@@ -1643,6 +1643,19 @@ choose_other_zone:
 	}
 
 retry:
+	/*
+	 * For random writes, retry from the zone chosen at the beginning using
+	 * the initial io_u random offset.
+	 */
+	if (td_random(td)) {
+		zone_unlock(z);
+		zone_lock(td, f, zb);
+		if (zbd_write_zone_get(td, f, zb))
+			return zb;
+		z = zb;
+		zone_idx = zbd_zone_idx(f, z);
+	}
+
 	/* Zone 'z' is full, so try to choose a new zone. */
 	for (i = f->io_size / zbdi->zone_size; i > 0; i--) {
 		zone_idx++;
-- 
2.49.0