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SubjectPATCH: raid1 on alpha
Driven by desperation :-) I have churned out a patch that works around a
long-standing problem with software RAID1 on Alpha platforms. gcc (at least
2.95 and 3.2) generated code for alpha apparently has a problem that is
exercised by the form of the loops in raid1_read_balance.

My admittedly lame patch, against 2.4.20, has been working for my SMP
Alphaserver 4100 for a couple of weeks now without ill effect. (Everything,
including root, on LVM on RAID1.) But I think I cut a corner or two in order
to get it working and keep it simple. I'm hoping a more artful/knowledgeable
programmer can massage this so it is acceptable for inclusion in the
mainline kernels.

Cheers,

Scott Bailey
scott.bailey@eds.com | rscottbailey@comcast.net


--- linux-2.4.20/drivers/md/raid1.c 2003-01-17 17:43:45.000000000 -0500
+++ linux-2.4.20-rsb/drivers/md/raid1.c 2003-01-17 17:43:20.000000000 -0500
@@ -11,6 +11,7 @@
*
* Fixes to reconstruction by Jakob Østergaard" <jakob@ostenfeld.dk>
* Various fixes by Neil Brown <neilb@cse.unsw.edu.au>
+ * Spastic hacks on raid1_read_balance by Scott Bailey
<scott.bailey@eds.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
@@ -475,6 +476,8 @@
int new_disk = conf->last_used;
const int sectors = bh->b_size >> 9;
const unsigned long this_sector = bh->b_rsector;
+ const int max_disk = conf->raid_disks;
+ const int loop_disk = max_disk - 1; /* optimizer is stooopid */
int disk = new_disk;
unsigned long new_distance;
unsigned long current_distance;
@@ -488,22 +491,18 @@


/* make sure that disk is operational */
- while( !conf->mirrors[new_disk].operational) {
- if (new_disk <= 0) new_disk = conf->raid_disks;
- new_disk--;
- if (new_disk == disk) {
- /*
- * This means no working disk was found
- * Nothing much to do, lets not change anything
- * and hope for the best...
- */
-
- new_disk = conf->last_used;
+ /* RSB: if not, be simple-minded and choose first readable disk */

- goto rb_out;
+ if (!conf->mirrors[new_disk].operational) {
+ for (disk = loop_disk; disk >= 0; disk--) {
+ if ((conf->mirrors[disk].operational) &&
+ (!conf->mirrors[disk].write_only)) break;
}
+ if (disk < 0) /* If no usable disk was found */
+ goto rb_out;
+ new_disk = disk;
}
- disk = new_disk;
+
/* now disk == new_disk == starting point for search */

/*
@@ -519,35 +518,26 @@
* This is for kicking those idling disks so that
* they would find work near some hotspot.
*/
-
- if (conf->sect_count >= conf->mirrors[new_disk].sect_limit) {
- conf->sect_count = 0;

-#if defined(CONFIG_SPARC64) && (__GNUC__ == 2) && (__GNUC_MINOR__ == 92)
- /* Work around a compiler bug in egcs-2.92.11 19980921 */
- new_disk = *(volatile int *)&new_disk;
-#endif
- do {
- if (new_disk<=0)
- new_disk = conf->raid_disks;
- new_disk--;
- if (new_disk == disk)
- break;
- } while ((conf->mirrors[new_disk].write_only) ||
- (!conf->mirrors[new_disk].operational));
+ /*
+ * RSB: Instead of making a scan for some other disk explicitly, just
+ * mark the head position as being at start of the disk. Then the next
+ * loop checking for "close" disks is almost certain to pick somebody
+ * else. Except for hot spots at front of disk, sigh.
+ */

- goto rb_out;
+ if (conf->sect_count >= conf->mirrors[new_disk].sect_limit) {
+ conf->sect_count = 0;
+ conf->mirrors[new_disk].head_position = 0;
}

- current_distance = abs(this_sector -
- conf->mirrors[disk].head_position);
-
/* Find the disk which is closest */
+ /* current_distance is initialized to guaranteed worse-than-worst
+ so that we can simplify loop logic */
+
+ current_distance = conf->mirrors[0].sect_limit + 1;

- do {
- if (disk <= 0)
- disk = conf->raid_disks;
- disk--;
+ for (disk = loop_disk; disk >= 0; disk--) {

if ((conf->mirrors[disk].write_only) ||
(!conf->mirrors[disk].operational))
@@ -561,7 +551,7 @@
current_distance = new_distance;
new_disk = disk;
}
- } while (disk != conf->last_used);
+ }

rb_out:
conf->mirrors[new_disk].head_position = this_sector + sectors;
-
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