I have several servers installed with a bootable raid1 array. I noticed
that under high load, the load wasn't balanced over the 2 disks in
the array anymore - all reads went to just one of the disks.
The problem is in raid1.c:read_balance().
There's a check to see if the array is in sync:
/*
* Check if it if we can balance. We can balance on the whole
* device if no resync is going on, or below the resync window.
* We take the first readable disk when above the resync window.
*/
if (!conf->mddev->in_sync && (this_sector + sectors >= conf->next_resync)) {
Now if you write to the array, the array is marked not in sync by
md.c:md_write_start(). conf->next_resync is initialized to zero, so
that means read balancing doesn't work anymore.
Now I think there should be a separate flag called 'resync_in_progress'
that flags whether, well, a resync is in progress ;) and the whole
read_balance() function should be simplified as well since it tests
the same things about three times, but for now here's a simple one-liner
that fixes it:
--- linux-2.6.6/drivers/md/raid1.c.orig 2004-05-10 04:32:37.000000000 +0200
+++ linux-2.6.6/drivers/md/raid1.c 2004-06-10 01:16:00.000000000 +0200
@@ -1172,6 +1172,7 @@
mddev->recovery_cp = MaxSector;
conf->resync_lock = SPIN_LOCK_UNLOCKED;
+ conf->next_resync = mddev->size << 1;
init_waitqueue_head(&conf->wait_idle);
init_waitqueue_head(&conf->wait_resume);
Signed-Off-By: Miquel van Smoorenburg <miquels@xxxxxxxxxx>
(please keep me cc'ed, I'm not on the list)
Mike.
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