With zonemode=zbd, for random read operations with read_beyond_wp=0,
the zbd code will always adjust an I/O offset so that the I/O falls in
a non empty zone. The adjustment however always sets the I/O offset to
the start of the zone, resulting in a high device read cache hit rate
if the device has very few zones written.
Fix this by improving the randomness of the I/Os by adjusting the I/O
offset to a random value with the range of written data in the zone.
Signed-off-by: Damien Le Moal <damien.lemoal@xxxxxxx>
---
zbd.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/zbd.c b/zbd.c
index 56197693..a985e022 100644
--- a/zbd.c
+++ b/zbd.c
@@ -1179,7 +1179,15 @@ enum io_u_action zbd_adjust_block(struct thread_data *td, struct io_u *io_u)
io_u->buflen);
goto eof;
}
- io_u->offset = zb->start << 9;
+ range = ((zb->wp - zb->start) << 9) - io_u->buflen;
+ if (td_random(td) && range >= 0)
+ io_u->offset = (zb->start << 9) +
+ ((io_u->offset - (zb->start << 9)) %
+ (range + 1)) / min_bs * min_bs;
+ else
+ io_u->offset = zb->start << 9;
+ assert(zb->start << 9 <= io_u->offset);
+ assert(io_u->offset + io_u->buflen <= zb->wp << 9);
}
if ((io_u->offset + io_u->buflen) >> 9 > zb->wp) {
dprint(FD_ZBD, "%s: %lld + %lld > %" PRIu64 "\n",
--
2.17.1
