About RAID1/10 io barrier between normal io and resync/recovery io.

Hi Neil:
At present, there are barriers in raid1/10.About barrier,you described: "Sometimes we need to suspend IO while we do something else, either some resync/recovery, or reconfigure the array"
So when do normal request in func make_request, it call wait_barrier.And in sync_request it call raise_barrier.
Of course,there are some place to call raise_barrier to do other things.
But there, i only wanted to talk about normal io and resync/recovery.

1:I think you add io barrier is because raid1/10 didn't stripe like raid456.So it only for the situation which normal io and sync io had same content. Is that right?

2: If normal ios outside the resync/recovery window, i think there is no necessary to use io barrier.

I wanted to find the reason by reviewing the git log.But at present, git log of kernel is only after kernel 2.6.12. So i looked into the code about kernel 2.4.And found something like i thought. There were resync window. But why did you remove or modify ?

3: I think using resync window to control normal io is perfect but complicated.So i only used one positon which before mddev->curr_resync_completed.

The code is:

drivers/md/raid1.c +21 -5

index 8034fbd..c45a769 100644
@@ -234,6 +234,7 @@ static void call_bio_endio(struct r1bio *r1_bio)
 {
        struct bio *bio = r1_bio->master_bio;
        int done;
+       int skip_barrier = test_bit(R1BIO_SKIP_BARRIER, &r1_bio->state);
        struct r1conf *conf = r1_bio->mddev->private;
 
        if (bio->bi_phys_segments) {

@@ -253,7 +254,8 @@ static void call_bio_endio(struct r1bio *r1_bio)
                 * Wake up any possible resync thread that waits for the device
                 * to go idle.
                 */
-               allow_barrier(conf);
+               if (!skip_barrier)
+                       allow_barrier(conf);
        }
 }
 

@@ -1007,6 +1009,7 @@ static void make_request(struct mddev *mddev, struct bio * bio)
        int first_clone;
        int sectors_handled;
        int max_sectors;
+       int skip_barrier = 0;
 
        /*
         * Register the new request and wait if the reconstruction

@@ -1036,7 +1039,13 @@ static void make_request(struct mddev *mddev, struct bio * bio)
                finish_wait(&conf->wait_barrier, &w);
        }
 
-       wait_barrier(conf);
+       if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
+               (bio->bi_sector + bio->bi_size/512 <= 
+                mddev->curr_resync_completed))
+               skip_barrier = 1;
+
+       if (!skip_barrier)
+               wait_barrier(conf);
 
        bitmap = mddev->bitmap;
 

@@ -1053,6 +1062,8 @@ static void make_request(struct mddev *mddev, struct bio * bio)
        r1_bio->mddev = mddev;
        r1_bio->sector = bio->bi_sector;
 
+       if (skip_barrier)
+               set_bit(R1BIO_SKIP_BARRIER, &r1_bio->state);
        /* We might need to issue multiple reads to different
         * devices if there are bad blocks around, so we keep
         * track of the number of reads in bio->bi_phys_segments.

@@ -1229,10 +1240,15 @@ read_again:
                for (j = 0; j < i; j++)
                        if (r1_bio->bios[j])
                                rdev_dec_pending(conf->mirrors[j].rdev, mddev);
-               r1_bio->state = 0;
-               allow_barrier(conf);
+               if (!skip_barrier)
+                       r1_bio->state = 0;
+               else
+                       set_bit(R1BIO_SKIP_BARRIER, &r1_bio->state);
+               if (!skip_barrier)
+                       allow_barrier(conf);
                md_wait_for_blocked_rdev(blocked_rdev, mddev);
-               wait_barrier(conf);
+               if (!skip_barrier)
+                       wait_barrier(conf);
                goto retry_write;
        }
 

drivers/md/raid1.h +2 -0

index 0ff3715..fab9fda 100644
@@ -158,6 +158,8 @@ struct r1bio {
 #define        R1BIO_MadeGood 7
 #define        R1BIO_WriteError 8
 
+#define R1BIO_SKIP_BARRIER 9
+
 extern int md_raid1_congested(struct mddev *mddev, int bits);
 
 #endif

At present, the code only for normal io and sync io.But etc stop/remove disk /add disk/fix_read_error also used io barrier. I used fio to test.
The fio script is:
[global]
filename=/dev/md0
direct=1
bs=512K

[write]
rw=write
runtime=600

[read]
stonewall
rw=read
runtime=600

I firstly started resync/recovery for some times in order to normal io position is before curr_resync_completed. And i also keep the resync/recovery speed is about 30MB/s throught sys_speed_min.
The result is:
w/ above code:
write recovery/resync 30MB/S; fio 70MB/s
read recovery/resync 30MB/s; fio 90MB/s

w/o above code:
write recovery/resync 30MB/s; fio 29MB/s
read recovery/resync 30MB/s; fio 31MB/s

The result is remarkable.

Jianpeng