untrusted comment: signature from openbsd 6.0 base secret key
RWSho3oKSqgLQ/PABGAAj4tgJm8RG0U2uH4SUOLh2b3YBXPRVkCb9GC+SW3GVZWBWjtPU3Eus+/+baddglO+AwJuMPPuIyG9oAE=

OpenBSD 6.0 errata 21, May 1, 2017:

softraid(4) was unable to create usable concat volumes because
it always set the size of the volume to zero sectors.

Apply by doing:
   signify -Vep /etc/signify/openbsd-60-base.pub -x 021_softraid_concat.patch.sig \
       -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
   cd /usr/src/sys/arch/`machine`/conf
   KK=`sysctl -n kern.osversion | cut -d# -f1`
   config $KK
   cd ../compile/$KK
   make
   make install

Index: sys/dev/softraid_concat.c
===================================================================
RCS file: /cvs/src/sys/dev/softraid_concat.c,v
--- sys/dev/softraid_concat.c   12 Apr 2016 16:26:54 -0000      1.25
+++ sys/dev/softraid_concat.c   28 Apr 2017 23:33:07 -0000      1.26
@@ -60,11 +60,19 @@ int
sr_concat_create(struct sr_discipline *sd, struct bioc_createraid *bc,
    int no_chunk, int64_t coerced_size)
{
+       int i;
+
       if (no_chunk < 2) {
               sr_error(sd->sd_sc, "%s requires two or more chunks",
                   sd->sd_name);
               return EINVAL;
        }
+
+       sd->sd_meta->ssdi.ssd_size = 0;
+       for (i = 0; i < no_chunk; i++) {
+               sd->sd_meta->ssdi.ssd_size +=
+                   sd->sd_vol.sv_chunks[i]->src_size;
+       }

       return sr_concat_init(sd);
}