Re: raid1 data corruption during resync

On Sep 2, 2014, at 4:55 PM, NeilBrown [off-list ref] wrote:

On Tue, 2 Sep 2014 15:07:26 -0700 Eivind Sarto [off-list ref] wrote:

quoted

On Sep 2, 2014, at 12:24 PM, Brassow Jonathan [off-list ref] wrote:

quoted

On Aug 29, 2014, at 2:29 PM, Eivind Sarto wrote:

quoted

I am seeing occasional data corruption during raid1 resync.
Reviewing the raid1 code, I suspect that commit 79ef3a8aa1cb1523cc231c9a90a278333c21f761 introduced a bug.
Prior to this commit raise_barrier() used to wait for conf->nr_pending to become zero.  It no longer does this.
It is not easy to reproduce the corruption, so I wanted to ask about the following potential fix while I am still testing it.
Once I validate that the fix indeed works, I will post a proper patch.
Do you have any feedback?

— drivers/md/raid1.c	2014-08-22 15:19:15.000000000 -0700

+++ /tmp/raid1.c	2014-08-29 12:07:51.000000000 -0700

@@ -851,7 +851,7 @@ static void raise_barrier(struct r1conf 

	 *    handling.
	 */
	wait_event_lock_irq(conf->wait_barrier,
-			    !conf->array_frozen &&
+			    !conf->array_frozen && !conf->nr_pending &&
			    conf->barrier < RESYNC_DEPTH &&
			    (conf->start_next_window >=
			     conf->next_resync + RESYNC_SECTORS),

This patch does not work - at least, it doesn't fix the issues I'm seeing.  My system hangs (in various places, like the resync thread) after commit 79ef3a8.  When testing this patch, I also added some code to dm-raid.c to allow me to print-out some of the variables when I encounter a problem.  After applying this patch and printing the variables, I see:
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: start_next_window = 12288
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: current_window_requests = -46
5257
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: next_window_requests = -11562
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: nr_pending = 0
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: nr_waiting = 0
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: nr_queued  = 0
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: barrier = 1
Sep  2 14:04:15 bp-01 kernel: device-mapper: raid: array_frozen = 0

Some of those values look pretty bizarre to me and suggest the accounting is pretty messed up.

brassow

After reviewing commit 79ef3a8aa1cb1523cc231c9a90a278333c21f761 I notice that wait_barrier() will now only exclude writes.  User reads are not excluded even if the fall within the resync window.
The old implementation used to exclude both reads and writes while resync-IO is active.
Could this be a cause of data corruption?

Could be.
From read_balance:

	if (conf->mddev->recovery_cp < MaxSector &&
	    (this_sector + sectors >= conf->next_resync))
		choose_first = 1;
	else
		choose_first = 0;

This used to be safe because a read immediately before next_resync would wait
until all resync requests completed.  But now that read requests don't block
it isn't safe.
Probably best to make this:

   choose_first = (conf->mddev->recovery_cp < this_sector + sectors);

Can you test that?

Thanks,
NeilBrown

I’ll give it a try.
-eivind

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