On Mar 20, 2013, at 6:04 PM, NeilBrown wrote:

On Wed, 20 Mar 2013 15:56:03 -0500 Brassow Jonathan [off-list ref]
wrote:

quoted

On Mar 19, 2013, at 9:46 PM, NeilBrown wrote:

quoted

On Tue, 19 Mar 2013 16:15:35 -0500 Brassow Jonathan [off-list ref]
wrote:

quoted

On Mar 17, 2013, at 6:49 PM, NeilBrown wrote:

quoted

On Wed, 13 Mar 2013 12:29:24 -0500 Jonathan Brassow [off-list ref]
wrote:

quoted

Neil,

I've noticed that when too many devices fail in a RAID arrary that
addtional I/O will hang, yielding an endless supply of:
Mar 12 11:52:53 bp-01 kernel: Buffer I/O error on device md1, logical block 3
Mar 12 11:52:53 bp-01 kernel: lost page write due to I/O error on md1
Mar 12 11:52:53 bp-01 kernel: sector=800 i=3           (null)           (null)  
      (null)           (null) 1

This is the third report in as many weeks that mentions that WARN_ON.
The first two where quite different causes.
I think this one is the same as the first one, which means it would be fixed
by  
   md/raid5: schedule_construction should abort if nothing to do.

which is commit 29d90fa2adbdd9f in linux-next.

Sorry, I don't see this commit in linux-next:
(the "for-next" branch of) git://github.com/neilbrown/linux.git
or git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git

Where should I be looking?

Sorry, I probably messed up.
I meant this commit:
http://git.neil.brown.name/?p=md.git;a=commitdiff;h=ce7d363aaf1e28be8406a2976220944ca487e8ca

Yes, I found this patch in 'for-next'.  I tested 3.9.0-rc3 with and without this patch.  The good news is that my issue with RAID5 appears to be fixed with this patch.  To test, I simply created a 1GB RAID array, let it sync, killed all of the devices and then issued a 40M write request (4M block size).  Before the patch, I would see the kernel warnings and it would take 7+ minutes to finish the 40M write.  After the patch, I don't see the kernel warnings or call traces and it takes < 1 sec to finish the 40M write.  That's good.  Will this patch make it back to 3.[78]?

However, I also found that RAID1 can take 2.5 min to perform the write and RAID10 can take 9+ min.  Hung task messages with call traces and many many errors are the result.  This is bad.  I haven't figured out why these are so slow yet.

What happens if you take RAID out of the picture?
i.e. write to a single device, then "kill" that device, then try issuing a
40M write request to it.

If that takes 2.5 minutes to resolve, then I think it is correct for RAID1 to
also take 2.5 minutes to resolve. 
If it resolves much more quickly than it does with RAID1, then that is a
problem we should certainly address.

The test is a little different because once you offline a device, you can't open it.  So, I had to start I/O and then kill the device.  I still get 158MB/s - 3 orders of magnitude faster than RAID1.  Besides, if RAID10 takes 9+ minutes to complete, we'd still have something to fix.  I have also tested this with an "error" device and it also returns in sub-second time.

 brassow

[root@bp-01 ~]# off.sh sda
Turning off sda
[root@bp-01 ~]# dd if=/dev/zero of=/dev/sda1 bs=4M count=10
dd: opening `/dev/sda1': No such device or address
[root@bp-01 ~]# on.sh sda
Turning on sda
[root@bp-01 ~]# dd if=/dev/zero of=/dev/sda bs=4M count=1000 &
[1] 5203
[root@bp-01 ~]# off.sh sda
Turning off sda
[root@bp-01 ~]# 1000+0 records in
1000+0 records out
4194304000 bytes (4.2 GB) copied, 26.5564 s, 158 MB/s