[PATCH] Re: User space RAID-6 access

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

> test_stripe assumes that the data starts at the start of each device.
> AS you are using 1.2 metadata (the default), data starts about 1M in to
> the device (I think - you can check with --examine)
> 
> You could fix test_stripe to put the right value in the 'offsets' array,
> or you could create the array with 1.0 or 0.90 metadata.

Hi Neil,

thanks for the info, maybe this should be a second patch.

In the meantime, please find attached a patch to restripe.c
of mdadm 3.2 (latest, I hope).

This should add the functionality to detect, in RAID-6,
which of the disks potentially has problems, in case of
parity errors.

Some checks take place in order to avoid false positives,
I hope these are correct and enough.

I'm not 100% happy of the interface (too much redundancy),
but for the time being it could be OK.

Of course, any improvement is welcome.

Please consider to include these changes to the next mdadm
whatever release.

bye,

--- restripe.c.org	2011-01-13 05:52:15.000000000 +0100
+++ restripe.c	2011-02-07 22:51:01.539471472 +0100
@@ -285,10 +285,13 @@
 uint8_t raid6_gfexp[256];
 uint8_t raid6_gfinv[256];
 uint8_t raid6_gfexi[256];
+uint8_t raid6_gflog[256];
+uint8_t raid6_gfilog[256];
 void make_tables(void)
 {
 	int i, j;
 	uint8_t v;
+	uint32_t b, log;
 
 	/* Compute multiplication table */
 	for (i = 0; i < 256; i++)
@@ -312,6 +315,19 @@
 	for (i = 0; i < 256; i ++)
 		raid6_gfexi[i] = raid6_gfinv[raid6_gfexp[i] ^ 1];
 
+	/* Compute log and inverse log */
+	/* Modified code from: http://web.eecs.utk.edu/~plank/plank/papers/CS-96-332.html */
+	b = 1;
+	raid6_gflog[0] = 0;
+	raid6_gfilog[255] = 0;
+
+	for (log = 0; log < 255; log++) {
+	  raid6_gflog[b] = (uint8_t) log;
+	  raid6_gfilog[log] = (uint8_t) b;
+	  b = b << 1;
+	  if (b & 256) b = b ^ 0435;
+	}
+
 	tables_ready = 1;
 }
 
@@ -387,6 +403,65 @@
 	}
 }
 
+/* Try to find out if a specific disk has a problem */
+int raid6_check_disks(int data_disks, int start, int chunk_size, int level, int layout, int diskP, int diskQ, char *p, char *q, char **stripes)
+{
+  int i;
+  int data_id, diskD;
+  uint8_t Px, Qx;
+  int curr_broken_disk = -1;
+  int prev_broken_disk = -1;
+  int broken_status = 0;
+
+  for(i = 0; i < chunk_size; i++) {
+    Px = (uint8_t)stripes[diskP][i] ^ (uint8_t)p[i];
+    Qx = (uint8_t)stripes[diskQ][i] ^ (uint8_t)q[i];
+
+    if((Px != 0) && (Qx == 0)) {
+      curr_broken_disk = diskP;
+    }
+
+    if((Px == 0) && (Qx != 0)) {
+      curr_broken_disk = diskQ;
+    }
+
+    if((Px != 0) && (Qx != 0)) {
+      data_id = (raid6_gflog[Qx] - raid6_gflog[Px]) & 0xFF;
+      diskD = geo_map(data_id, start/chunk_size, data_disks + 2, level, layout);
+      curr_broken_disk = diskD;
+    }
+
+    if((Px == 0) && (Qx == 0)) {
+      curr_broken_disk = curr_broken_disk;
+    }
+
+    switch(broken_status) {
+    case 0:
+      if(curr_broken_disk != -1) {
+	prev_broken_disk = curr_broken_disk;
+	broken_status = 1;
+      }
+      break;
+
+    case 1:
+      if(curr_broken_disk != prev_broken_disk) {
+	broken_status = 2;
+      }
+      if(curr_broken_disk >= data_disks + 2) {
+	broken_status = 2;
+      }
+      break;
+
+    case 2:
+    default:
+      curr_broken_disk = prev_broken_disk = -2;
+      break;
+    }
+  }
+
+  return curr_broken_disk;
+}
+
 /* Save data:
  * We are given:
  *  A list of 'fds' of the active disks.  Some may be absent.
@@ -673,7 +748,12 @@
 	char *q = malloc(chunk_size);
 
 	int i;
+	int diskP, diskQ;
 	int data_disks = raid_disks - (level == 5 ? 1: 2);
+
+	if (!tables_ready)
+		make_tables();
+
 	for ( i = 0 ; i < raid_disks ; i++)
 		stripes[i] = stripe_buf + i * chunk_size;
 
@@ -693,18 +773,25 @@
 		switch(level) {
 		case 6:
 			qsyndrome(p, q, (uint8_t**)blocks, data_disks, chunk_size);
-			disk = geo_map(-1, start/chunk_size, raid_disks,
+			diskP = geo_map(-1, start/chunk_size, raid_disks,
 				       level, layout);
-			if (memcmp(p, stripes[disk], chunk_size) != 0) {
-				printf("P(%d) wrong at %llu\n", disk,
+			if (memcmp(p, stripes[diskP], chunk_size) != 0) {
+				printf("P(%d) wrong at %llu\n", diskP,
 				       start / chunk_size);
 			}
-			disk = geo_map(-2, start/chunk_size, raid_disks,
+			diskQ = geo_map(-2, start/chunk_size, raid_disks,
 				       level, layout);
-			if (memcmp(q, stripes[disk], chunk_size) != 0) {
-				printf("Q(%d) wrong at %llu\n", disk,
+			if (memcmp(q, stripes[diskQ], chunk_size) != 0) {
+				printf("Q(%d) wrong at %llu\n", diskQ,
 				       start / chunk_size);
 			}
+			disk = raid6_check_disks(data_disks, start, chunk_size, level, layout, diskP, diskQ, p, q, stripes);
+			if(disk >= 0) {
+			  printf("Possible failed disk: %d\n", disk);
+			}
+			if(disk == -2) {
+			  printf("Failure detected, but disk unknown\n");
+			}
 			break;
 		}
 		length -= chunk_size;

-- 

piergiorgio