dont sort io for thin disks (or ssds)

[David Gwynne]

now that we know if the blocks under a disk are not in the order
the os uses them, we dont have to bother sorting io.

ok?

Index: sd.c
===================================================================
RCS file: /cvs/src/sys/scsi/sd.c,v
retrieving revision 1.235
diff -u -p -r1.235 sd.c
--- sd.c        11 Jul 2011 06:26:09 -0000      1.235
+++ sd.c        11 Jul 2011 12:17:40 -0000
@@ -163,7 +163,7 @@ sdattach(struct device *parent, struct d
        int sd_autoconf = scsi_autoconf | SCSI_SILENT |
            SCSI_IGNORE_ILLEGAL_REQUEST | SCSI_IGNORE_MEDIA_CHANGE;
        struct dk_cache dkc;
-       int error, result;
+       int error, result, sortby = BUFQ_DEFAULT;
 
        SC_DEBUG(sc_link, SDEV_DB2, ("sdattach:\n"));
 
@@ -174,12 +174,6 @@ sdattach(struct device *parent, struct d
        sc_link->interpret_sense = sd_interpret_sense;
        sc_link->device_softc = sc;
 
-       /*
-        * Initialize disk structures.
-        */
-       sc->sc_dk.dk_name = sc->sc_dev.dv_xname;
-       bufq_init(&sc->sc_bufq, BUFQ_DEFAULT);
-
        if ((sc_link->flags & SDEV_ATAPI) && (sc_link->flags & SDEV_REMOVABLE))
                sc_link->quirks |= SDEV_NOSYNCCACHE;
 
@@ -233,8 +227,10 @@ sdattach(struct device *parent, struct d
                    sc->sc_dev.dv_xname,
                    dp->disksize / (1048576 / dp->secsize), dp->secsize,
                    dp->disksize);
-               if (ISSET(sc->flags, SDF_THIN))
+               if (ISSET(sc->flags, SDF_THIN)) {
+                       sortby = BUFQ_FIFO;
                        printf(", thin");
+               }
                printf("\n");
                break;
 
@@ -248,6 +244,15 @@ sdattach(struct device *parent, struct d
 #endif
        }
 
+       /*
+        * Initialize disk structures.
+        */
+       sc->sc_dk.dk_name = sc->sc_dev.dv_xname;
+       bufq_init(&sc->sc_bufq, sortby);
+
+       /*
+        * Enable write cache by default.
+        */
        memset(&dkc, 0, sizeof(dkc));
        if (sd_ioctl_cache(sc, DIOCGCACHE, &dkc) == 0 && dkc.wrcache == 0) {
                dkc.wrcache = 1;