raid6 rebuild

raid6 rebuild

[Lennert Buytenhek]

(please CC on replies, not subscribed to linux-raid@)

Hi!

While my RAID6 array was rebuilding after one disk had failed (which
I replaced), a second disk failed[*], and this caused the rebuild
process to start over from the beginning.

Why would the rebuild need to start over from the beginning in this
case?  Why couldn't it just continue from where it was?


thanks,
Lennert

[*] probably an entirely defective batch of 14 Samsung Spinpoint 500G disks

Re: raid6 rebuild

[Dan Williams]

On 4/4/07, Lennert Buytenhek <buytenh@wantstofly.org> wrote:
> (please CC on replies, not subscribed to linux-raid@)
>
> Hi!
>
> While my RAID6 array was rebuilding after one disk had failed (which
> I replaced), a second disk failed[*], and this caused the rebuild
> process to start over from the beginning.
>
> Why would the rebuild need to start over from the beginning in this
> case?  Why couldn't it just continue from where it was?
>
I believe it is because raid5 and raid6 share the same error handler
which sets MD_RECOVERY_ERR after losing any disk.  It should probably
not set this flag in 1-disk lost raid6 case, but I might be
overlooking something else.

>
> thanks,
> Lennert
>

Dan

Re: raid6 rebuild

[Lennert Buytenhek]

On Wed, Apr 04, 2007 at 08:22:00PM -0700, Dan Williams wrote:

> >While my RAID6 array was rebuilding after one disk had failed (which
> >I replaced), a second disk failed[*], and this caused the rebuild
> >process to start over from the beginning.
> >
> >Why would the rebuild need to start over from the beginning in this
> >case?  Why couldn't it just continue from where it was?
> 
> I believe it is because raid5 and raid6 share the same error handler
> which sets MD_RECOVERY_ERR after losing any disk.  It should probably
> not set this flag in 1-disk lost raid6 case, but I might be
> overlooking something else.

Right, so you're saying that it's probably a bug rather than an
intentional 'feature'?

Re: raid6 rebuild

[Bill Davidsen]

Lennert Buytenhek wrote:
> On Wed, Apr 04, 2007 at 08:22:00PM -0700, Dan Williams wrote:
>
>   
>>> While my RAID6 array was rebuilding after one disk had failed (which
>>> I replaced), a second disk failed[*], and this caused the rebuild
>>> process to start over from the beginning.
>>>
>>> Why would the rebuild need to start over from the beginning in this
>>> case?  Why couldn't it just continue from where it was?
>>>       
>> I believe it is because raid5 and raid6 share the same error handler
>> which sets MD_RECOVERY_ERR after losing any disk.  It should probably
>> not set this flag in 1-disk lost raid6 case, but I might be
>> overlooking something else.
>>     
>
> Right, so you're saying that it's probably a bug rather than an
> intentional 'feature'?

No, I would say it's a "reviewable design decision" To be pedantic (as I 
often am), a bug really means that unintended results are generated. In 
this case I think the code functions as intended, but might be able to 
safely take some other action.

I confess, I would feel safer with my data if the rebuild started over, 
I would like to be sure that when it (finally) finishes the data are 
valid. If you replaced the 2nd drive, then a full rebuild would be 
required in any case, to get ALL drives valid.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: raid6 rebuild

[Lennert Buytenhek]

On Thu, Apr 05, 2007 at 09:54:14AM -0400, Bill Davidsen wrote:

> I confess, I would feel safer with my data if the rebuild started
> over, I would like to be sure that when it (finally) finishes the
> data are valid.

With disk #3 about to die, I'd have felt safer if it first finished
rebuilding the replacement disk for failed disk #1 (that rebuild had
almost completed at that point), safeguarding the array against a
third disk failure.

Yeah, I know, you're not supposed to lose three disks within one day.


> If you replaced the 2nd drive, then a full rebuild would be required
> in any case, to get ALL drives valid.

You could finish the last little bit of the resync of the replacement
for failed disk #1 (by looking at P+Q), and then re-sync the replacement
for failed disk #2 (by looking only at P..)

Re: raid6 rebuild

[Dan Williams]

On 4/5/07, Lennert Buytenhek <buytenh@wantstofly.org> wrote:
> On Thu, Apr 05, 2007 at 09:54:14AM -0400, Bill Davidsen wrote:
>
> > I confess, I would feel safer with my data if the rebuild started
> > over, I would like to be sure that when it (finally) finishes the
> > data are valid.
>
> With disk #3 about to die, I'd have felt safer if it first finished
> rebuilding the replacement disk for failed disk #1 (that rebuild had
> almost completed at that point), safeguarding the array against a
> third disk failure.
>
I agree, the current arrangement seems to throw away a significant
amount of work.  Yes, you will need to resync when re-adding the
second disk, but in the meantime might as well try to get a redundant
mode at all costs.

> Yeah, I know, you're not supposed to lose three disks within one day.
>
>
> > If you replaced the 2nd drive, then a full rebuild would be required
> > in any case, to get ALL drives valid.
>
> You could finish the last little bit of the resync of the replacement
> for failed disk #1 (by looking at P+Q), and then re-sync the replacement
> for failed disk #2 (by looking only at P..)
> -

--
Dan

Re: raid6 rebuild

[Neil Brown]

On Thursday April 5, dan.j.williams@intel.com wrote:
> On 4/5/07, Lennert Buytenhek <buytenh@wantstofly.org> wrote:
> > On Thu, Apr 05, 2007 at 09:54:14AM -0400, Bill Davidsen wrote:
> >
> > > I confess, I would feel safer with my data if the rebuild started
> > > over, I would like to be sure that when it (finally) finishes the
> > > data are valid.
> >
> > With disk #3 about to die, I'd have felt safer if it first finished
> > rebuilding the replacement disk for failed disk #1 (that rebuild had
> > almost completed at that point), safeguarding the array against a
> > third disk failure.
> >
> I agree, the current arrangement seems to throw away a significant
> amount of work.  Yes, you will need to resync when re-adding the
> second disk, but in the meantime might as well try to get a redundant
> mode at all costs.

Yes, I think you are right.
If you want it to restart from the beginning you can alway abort the
current resync by 'echo idle > sync_action'.
The question is: is it really as simple to do as it sounds.
I seem to remember that aborting the recovery on any error was any
easy way to avoid some nasty race, but I have no idea what the race
was.
One would need the enumerate all the interesting cases and make sure
they will all work as expected.  I cannot think of an problems
immediately but that doesn't mean there aren't any...

It is now on my todo list...

NeilBrown

Re: raid6 rebuild

[Gordon Henderson]

On Wed, 4 Apr 2007, Lennert Buytenhek wrote:

> (please CC on replies, not subscribed to linux-raid@)
>
> Hi!
>
> While my RAID6 array was rebuilding after one disk had failed (which
> I replaced), a second disk failed[*], and this caused the rebuild
> process to start over from the beginning.
>
> Why would the rebuild need to start over from the beginning in this
> case?  Why couldn't it just continue from where it was?

I can't answer your question, just make the comment: Hurrah for RAID-6!

I presume the restarted rebuild went OK?

> [*] probably an entirely defective batch of 14 Samsung Spinpoint 500G disks

Lets hope not... Keep checking those SMART values...

Gordon

Re: raid6 rebuild

[Lennert Buytenhek]

On Thu, Apr 05, 2007 at 07:03:08AM +0100, Gordon Henderson wrote:

> > While my RAID6 array was rebuilding after one disk had failed (which
> > I replaced), a second disk failed[*], and this caused the rebuild
> > process to start over from the beginning.
> >
> > Why would the rebuild need to start over from the beginning in this
> > case?  Why couldn't it just continue from where it was?
> 
> I can't answer your question, just make the comment: Hurrah for RAID-6!

Agreed, although I'm still only just in the testing phase with this
array.  It isn't in production yet, and I don't have any important data
on the array that I don't have backups of.


> I presume the restarted rebuild went OK?

Yep, it had just finished when I just woke up, so now it's back from
double-degraded to degraded.  Just in time for the third disk to start
failing, which will probably fail during the rebuild of the second
failed disk after I swap that one out in a couple of minutes..


> > [*] probably an entirely defective batch of 14 Samsung Spinpoint
> > 500G disks
> 
> Lets hope not... Keep checking those SMART values...

Failed disk #2 still reports a SMART status of "PASSED"..

Re: raid6 rebuild

[Gordon Henderson]

On Thu, 5 Apr 2007, Lennert Buytenhek wrote:

>>> [*] probably an entirely defective batch of 14 Samsung Spinpoint
>>> 500G disks
>>
>> Lets hope not... Keep checking those SMART values...
>
> Failed disk #2 still reports a SMART status of "PASSED"..

Hmmm.. I'd be tempted to double check your hardware + kernel versions then 
- I did have an older SCSI array on an older server (dual 500Mhz Xeons!) 
give me the occasional sector read error, but when going back and using 
badblocks (read only) on the disk, it checked out just fine... I never got 
to the bottom of it, but it only started happening when I went from a 2.4 
kernel where it was configures as 2 x 4-disk RAID-5 arrays to a 2.6 one 
where it was configures as a single 8 drive RAID-6 array with the new 
Adaptec SCSI driver... (My suspicions were that of old hardware combined 
with new drivers and a slight motherboard timing issue, as the R6 code 
might well drive the underlying hardware "harder" than when it was in dual 
R5 mode, but it was retired before it became a serious issue)

Gordon

Re: raid6 rebuild

[Lennert Buytenhek]

On Thu, Apr 05, 2007 at 09:15:38AM +0100, Gordon Henderson wrote:

> >>>[*] probably an entirely defective batch of 14 Samsung Spinpoint
> >>>500G disks
> >>
> >>Lets hope not... Keep checking those SMART values...
> >
> >Failed disk #2 still reports a SMART status of "PASSED"..
> 
> Hmmm.. I'd be tempted to double check your hardware + kernel versions
> then 

I think in this case it's more of a disagreement between me and the
disk about whether certain levels and patterns of bad sectors are
acceptable or not.

dd'ing the disk to /dev/null keeps reporting new UncorrectableError
sectors, which overwriting with zeroes then supposedly fixes (judging
by the increasing Reallocated_Sector_Ct), but a subsequent read from
the very same sectors reports the very same UncorrectableError errors
again, plus new errors from other sectors.

The SMART short offline tests also report "Completed: read failure".

Meanwhile, the disk still feels that that its SMART status is PASSED.

(FYI, these are Samsung Spinpoint T166S HD501LJ disks with serial
numbers in the range S0VVJ1KP3xxxxx)

Re: raid6 rebuild

[Andre Noll]

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On 21:46, Lennert Buytenhek wrote:

> While my RAID6 array was rebuilding after one disk had failed (which
> I replaced), a second disk failed[*], and this caused the rebuild
> process to start over from the beginning.
> 
> Why would the rebuild need to start over from the beginning in this
> case?  Why couldn't it just continue from where it was?

I think this is because an easier algorithm may be used to rebuild from
a one disk failure than what is needed in case of a two disk failure.
In case of a one disk failure you can simply do the following:

rebuild P or Q: Just recompute it from the data disks
rebuild data: xor remaining data and P as in raid5 (in fact, the raid5
code is used in this case)

It's only the rebuild from a two disk failure that requires deeper
and more expensive math.

So if a second disk dies during the rebuild, the raid6 code must
switch from the easy algorithm to the "difficult" algorithm to finish
rebuilding the first failed disk. Theoretically this could be done
on the fly, but I think the current code doesn't do this now.

Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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