Hi,
according to the source, ZFS aims to have around 200 metaslabs per vdev:
1141 void
1142 vdev_init(vdev_t *vd, uint64_t txg)
1143 {
1144 /*
1145 * Aim for roughly 200 metaslabs per vdev.
1146 */
1147 vd->vdev_ms_shift = highbit(vd->vdev_asize / 200);
1148 vd->vdev_ms_shift = MAX(vd->vdev_ms_shift, SPA_MAXBLOCKSHIFT);
In practice I noticed that number of metaslabs tends to be around 128.
To make it closer to 200 (256?), I think line 1147 needs to be
1147 vd->vdev_ms_shift = highbit(vd->vdev_asize / 200) - 1;
With the best regards,
Victor