zbd: Improve read randomness

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 hits a
non empty zone. However, the adjustment 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 few zones written.

Improve randomness of read I/Os by adjusting the I/O offset to a random
value within the range of written data of the chosen zone. Also ensure
that the modified I/O does not cross over the zone wp position by
adjusting its size. Doing so, the search for a new zone is now done only
if the target zone cannot accomodate an I/O of at least min_bs bytes.

The comments have also been changed to better explain the code.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>

diff --git a/zbd.c b/zbd.c
index 561976939298f9ad98991e9c5c3098a31049baf3..93f14410797a8db20c1496aaf7fab6640f6353d0 100644 (file)
--- a/zbd.c
+++ b/zbd.c
@@ -1119,7 +1119,7 @@ enum io_u_action zbd_adjust_block(struct thread_data *td, struct io_u *io_u)
 {
        const struct fio_file *f = io_u->file;
        uint32_t zone_idx_b;
-       struct fio_zone_info *zb, *zl;
+       struct fio_zone_info *zb, *zl, *orig_zb;
        uint32_t orig_len = io_u->buflen;
        uint32_t min_bs = td->o.min_bs[io_u->ddir];
        uint64_t new_len;
@@ -1132,6 +1132,7 @@ enum io_u_action zbd_adjust_block(struct thread_data *td, struct io_u *io_u)
        assert(io_u->buflen);
        zone_idx_b = zbd_zone_idx(f, io_u->offset);
        zb = &f->zbd_info->zone_info[zone_idx_b];
+       orig_zb = zb;
 
        /* Accept the I/O offset for conventional zones. */
        if (zb->type == BLK_ZONE_TYPE_CONVENTIONAL)
@@ -1153,21 +1154,14 @@ enum io_u_action zbd_adjust_block(struct thread_data *td, struct io_u *io_u)
                        goto accept;
                }
                /*
-                * Avoid reads past the write pointer because such reads do not
-                * hit the medium.
+                * Check that there is enough written data in the zone to do an
+                * I/O of at least min_bs B. If there isn't, find a new zone for
+                * the I/O.
                 */
                range = zb->cond != BLK_ZONE_COND_OFFLINE ?
-                       ((zb->wp - zb->start) << 9) - io_u->buflen : 0;
-               if (td_random(td) && range >= 0) {
-                       io_u->offset = (zb->start << 9) +
-                               ((io_u->offset - (zb->start << 9)) %
-                                (range + 1)) / min_bs * min_bs;
-                       assert(zb->start << 9 <= io_u->offset);
-                       assert(io_u->offset + io_u->buflen <= zb->wp << 9);
-                       goto accept;
-               }
-               if (zb->cond == BLK_ZONE_COND_OFFLINE ||
-                   (io_u->offset + io_u->buflen) >> 9 > zb->wp) {
+                       (zb->wp - zb->start) << 9 : 0;
+               if (range < min_bs ||
+                   ((!td_random(td)) && (io_u->offset + min_bs > zb->wp << 9))) {
                        pthread_mutex_unlock(&zb->mutex);
                        zl = &f->zbd_info->zone_info[zbd_zone_idx(f,
                                                f->file_offset + f->io_size)];
@@ -1179,14 +1173,39 @@ 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;
+                       /*
+                        * zbd_find_zone() returned a zone with a range of at
+                        * least min_bs.
+                        */
+                       range = (zb->wp - zb->start) << 9;
+                       assert(range >= min_bs);
+
+                       if (!td_random(td))
+                               io_u->offset = zb->start << 9;
                }
-               if ((io_u->offset + io_u->buflen) >> 9 > zb->wp) {
-                       dprint(FD_ZBD, "%s: %lld + %lld > %" PRIu64 "\n",
-                              f->file_name, io_u->offset, io_u->buflen,
-                              zb->wp);
-                       goto eof;
+               /*
+                * Make sure the I/O is within the zone valid data range while
+                * maximizing the I/O size and preserving randomness.
+                */
+               if (range <= io_u->buflen)
+                       io_u->offset = zb->start << 9;
+               else if (td_random(td))
+                       io_u->offset = (zb->start << 9) +
+                               ((io_u->offset - (orig_zb->start << 9)) %
+                                (range - io_u->buflen)) / min_bs * min_bs;
+               /*
+                * Make sure the I/O does not cross over the zone wp position.
+                */
+               new_len = min((unsigned long long)io_u->buflen,
+                             (unsigned long long)(zb->wp << 9) - io_u->offset);
+               new_len = new_len / min_bs * min_bs;
+               if (new_len < io_u->buflen) {
+                       io_u->buflen = new_len;
+                       dprint(FD_IO, "Changed length from %u into %llu\n",
+                              orig_len, io_u->buflen);
                }
+               assert(zb->start << 9 <= io_u->offset);
+               assert(io_u->offset + io_u->buflen <= zb->wp << 9);
                goto accept;
        case DDIR_WRITE:
                if (io_u->buflen > (f->zbd_info->zone_size << 9))