* Re: the dreaded double disk failure
2005-01-13 8:37 ` Guy
@ 2005-01-13 8:47 ` Mike Hardy
2005-01-15 3:59 ` Mike Hardy
1 sibling, 0 replies; 5+ messages in thread
From: Mike Hardy @ 2005-01-13 8:47 UTC (permalink / raw)
To: Guy; +Cc: linux-raid
Thanks for responding, Guy - I appreciate it
Guy wrote:
> You should do something like a nightly dd of every disk! Then you would
I have smartd set up to do a nightly short scan of all drives, and a
weekly full scan. I may alter that to be a full scan of all drives once
a day after this. I do test them though, and frequently
> bad block I fail the disk then overwrite the disk/partition with a dd
> command. This causes the disk to re-map the bad block to a spare block.
> Then I test the disk with another dd read command. Once I am sure the disk
> is good, I add it back to the array. All of this is a real pain in the @$$.
> Some people just fail the disk, then add it back in. They just let the
> re-sync cause the disk to re-map the bad block. I guess I feel more
> in-control my way.
I typically just fail the disk and re-add it - but sometimes I do the
manual fail/dd/add way. You're right its a pain in the butt, but its
moot at this point because I've got two bad spots in one raid 5
> If raidreconf did not finish, I think you should expect major data loss!
> If raidreconf did not finish, stop here and ignore any advice below!
I wasn't terribly clear (its been a long day) - the raidreconf failure
was on a separate machine containing my backup array. This is where my
main array would rsync too nightly, if it weren't hosed. I gave up on it
though (funnily enough, it got a bad sector too, sigh), and just
re-built that array from scratch, clean, after full scans on all drives.
So my backup array is fresh and in sync and ready for data if I can get
the data.
> You have more than 1 option.
That's a good spot to be in
> OPTION ONE:
> If you assemble the array with 1 missing disk and no spare, it will not
> attempt to re-build or re-sync. It will just be fine until it finds the bad
> block as you said.
>
> So, I think your plan will work. But I think you may need to assemble 3
> times before you have all of your data! In each case, when you determine
> which file is on a bad block, delete the file after you get a good copy,
> then the next time you will not have a read error on that file. I think
> this is what you meant, but not sure.
That is what I meant, and this is my most safe, but most tedious option
I think. I could do it in two restarts if I picked the right drives I
think, but its hard to say. Either way I thought it would work, and I'll
take this as a second on that idea.
> OPTION TWO:
> If you have an extra disk, you could use dd_rescue to make a copy of one of
> your bad disks. This will cause corruption related to the bad block. But
> it would get you going again. Then assemble your array with this "new" disk
> and the other bad disk as missing. Once you are sure your data is there you
> could add you missing disk and it will re-sync. The re-sync should cause
> the disk to re-map the bad block.
I am out of disks, and this seems like a painful option
> OPTION THREE:
> Another idea! Maybe risky! It scares me!
> But if I am correct, no data loss.
> For this to work, you must not use any tools to change any data on any of
> the 8 disks!!!!!!!!!!
> No attempts to repair the disks with dd or anything!!!!!
>
> Assemble your array with hdk1 missing. Then add hdk1 to the array, the
> array will start to re-sync. This re-sync should overwrite the disk with
> the same data that is all ready there. The re-sync should re-map the bad
> block and continue until hdo1 hits its bad block. At that time hdo1 will be
> kicked out, and the array will be down. But hdk1 should now be good since
> the data should still be on it. So, now assemble the array with hdo1
> missing, then add hdo1 and a re-sync will start, this should correct the bad
> block and the re-sync should finish, unless you have a third bad block.
> Each time you have a read error, just repeat the process with the disk that
> got the last read error as the missing disk, then add it to the array to
> start another re-sync.
Now that's an interesting thought. Is resync linear and monotonically
increasing across the LBA addresses of the drives? If so, I think this
might become the "standard" way to recover from multi-disk failures in
raid5, if you can assume the errors are on different stripes and you
know their locations (from smartctl, etc)
I'd love to hear Neil's take on this.
> OPTION FOUR: (not an option)
> A standalone tool to scan the disks and repair as you suggest would be real
> cool! It would just read test every disk until it finds a read error, then
> compute the missing data, then re-write it. Then continue on. It could
> also verify the parity and correct as needed. I don't think such a tool
> exists today.
This is intriguing as a potential contribution to the pool of utilities
out there, as I get a lot of benefit from it, and it'd be neat to put
something back. I'm still not sure if its possible though
Thanks again, and I'll see what other people have to say
-Mike
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: the dreaded double disk failure
2005-01-13 8:37 ` Guy
2005-01-13 8:47 ` Mike Hardy
@ 2005-01-15 3:59 ` Mike Hardy
2005-01-15 22:52 ` raid5 test array creator (was Re: the dreaded double disk failure) Mike Hardy
1 sibling, 1 reply; 5+ messages in thread
From: Mike Hardy @ 2005-01-15 3:59 UTC (permalink / raw)
To: linux-raid
I just had a better thought with regards to recovering from this
double-disk failure, that would actually help others (gasp!) and be
useful if/when the raid code starts asking a helper program to examine
read errors.
For those not keeping score at home, I've got an 8 disk raid5 array
where two of the disks have bad sectors, but the bad sectors are in very
different areas on each disk. Otherwise its in sync, and offline right now.
What I'm thinking of doing is writing a small (or, as small as possible,
anyway) perl program that can take a few command line arguments (like
the array construction information) and know how to read the data blocks
on the array, and calculate parity, as a baseline. If perl offends you,
sorry, I'm quicker at it than C by a long-shot, and I don't really care
about speed here, just speed of development.
The second step will be to add a new argument, namely the location of a
bad block on a component. The script should be able to examine all of
the blocks in the array around that block and calculate what data
*should* be in there, using parity if necessary.
It could be extended later to actually do parity checks perhaps, or
anything really, but my main goal is to write a script that understands
the on-disk layout of raid5 and can reconstruct data, optionally writing
the data back into the bad block (reallocating the sector and clearing
the problem).
If anyone's got pointers to useful raid5 docs, or similar
implementations, I'd love to see them. Otherwise I'm going to start
hacking around and see where I get.
-Mike
Guy wrote:
> Your static data could be on a bad block, you never know.
>
> You should do something like a nightly dd of every disk! Then you would
> stand a good chance of finding the bad block before md does. When I find a
> bad block I fail the disk then overwrite the disk/partition with a dd
> command. This causes the disk to re-map the bad block to a spare block.
> Then I test the disk with another dd read command. Once I am sure the disk
> is good, I add it back to the array. All of this is a real pain in the @$$.
> Some people just fail the disk, then add it back in. They just let the
> re-sync cause the disk to re-map the bad block. I guess I feel more
> in-control my way.
>
> After I started testing my disks every night, I stopped getting bad blocks.
> Maybe blocks need to be read every so often to keep them working? Sounds
> stupid to me too! Maybe I have just been lucky!
>
> Ok, lecture over. :)
>
> If raidreconf did not finish, I think you should expect major data loss!
> If raidreconf did not finish, stop here and ignore any advice below!
>
> You have more than 1 option.
>
> OPTION ONE:
> If you assemble the array with 1 missing disk and no spare, it will not
> attempt to re-build or re-sync. It will just be fine until it finds the bad
> block as you said.
>
> So, I think your plan will work. But I think you may need to assemble 3
> times before you have all of your data! In each case, when you determine
> which file is on a bad block, delete the file after you get a good copy,
> then the next time you will not have a read error on that file. I think
> this is what you meant, but not sure.
>
> OPTION TWO:
> If you have an extra disk, you could use dd_rescue to make a copy of one of
> your bad disks. This will cause corruption related to the bad block. But
> it would get you going again. Then assemble your array with this "new" disk
> and the other bad disk as missing. Once you are sure your data is there you
> could add you missing disk and it will re-sync. The re-sync should cause
> the disk to re-map the bad block.
>
> OPTION THREE:
> Another idea! Maybe risky! It scares me!
> But if I am correct, no data loss.
> For this to work, you must not use any tools to change any data on any of
> the 8 disks!!!!!!!!!!
> No attempts to repair the disks with dd or anything!!!!!
>
> Assemble your array with hdk1 missing. Then add hdk1 to the array, the
> array will start to re-sync. This re-sync should overwrite the disk with
> the same data that is all ready there. The re-sync should re-map the bad
> block and continue until hdo1 hits its bad block. At that time hdo1 will be
> kicked out, and the array will be down. But hdk1 should now be good since
> the data should still be on it. So, now assemble the array with hdo1
> missing, then add hdo1 and a re-sync will start, this should correct the bad
> block and the re-sync should finish, unless you have a third bad block.
> Each time you have a read error, just repeat the process with the disk that
> got the last read error as the missing disk, then add it to the array to
> start another re-sync.
>
> I think the above should work regardless of which disk you use as the
> missing disk. But if you chose poorly, you will have 1 extra iteration of
> the whole process.
>
> OPTION FOUR: (not an option)
> A standalone tool to scan the disks and repair as you suggest would be real
> cool! It would just read test every disk until it finds a read error, then
> compute the missing data, then re-write it. Then continue on. It could
> also verify the parity and correct as needed. I don't think such a tool
> exists today.
>
> Whatever you choose, getting a second (or third) opinion can't hurt!
>
> Guy
>
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Mike Hardy
> Sent: Thursday, January 13, 2005 2:14 AM
> To: linux-raid@vger.kernel.org
> Subject: the dreaded double disk failure
>
>
> Alas, I've been bitten.
>
> Worse, it was after attempting to use raidreconf and having it trash the
> array with my backup on it instead of extending it. I know raidreconf is
> a use-at-your-own-risk tool, but it was the backup, so I didn't mind.
>
> Until I got this (partial mdadm -E output):
>
> Number Major Minor RaidDevice State
> this 7 91 1 7 active sync /dev/hds1
> 0 0 33 1 0 active sync /dev/hde1
> 1 1 34 1 1 active sync /dev/hdg1
> 2 2 56 1 2 active sync /dev/hdi1
> 3 3 57 1 3 faulty /dev/hdk1
> 4 4 88 1 4 active sync /dev/hdm1
> 5 5 89 1 5 faulty /dev/hdo1
> 6 6 90 1 6 active sync /dev/hdq1
> 7 7 91 1 7 active sync /dev/hds1
>
> /dev/hdk1 has at least one unreadable block around LBA 3,600,000 or so,
> and /dev/hdo1 has at least one unreadable blok around LBA 8,000,000 or so.
>
> Further, the array was resyncing (power failure due to construction, yes
> its been one of those days - but it was actually in sync) when the first
> bad block hit, but I know that all the data I care about was static at
> the time, so barring some fsck cleanup, all the important blocks should
> have correct parity.
>
> Which is to say that I think my data exists, its just a bit far away at
> the moment.
>
> The first question is, would you agree?
>
> Assuming its there, my general plan is to do this to get my data out:
>
> 1) resurrect the backup array
> 2) add one faulty drive to the array, with bad blocks there
> (an mdadm assemble with 7 of the 8, forced?)
> 3) start the backup, fully anticipating the read error and disk ejection
> 4) add the other faulty drive in, with bad blocks there
> (mdadm assemble with 7 of the 8, forced again?)
> 5) finish the backup
>
> The second question is, does that sound sane? Or is there a better way?
>
> Finally, to get the main array healthy, I'm going to take note of which
> files kicked out which drives, and clobber them with the backed up version.
>
> Alternately, how hard would it be to write a utility that inspected the
> array, took the LBA(s) of the bad block on one component, and
> reconstructed it for rewrite via parity. A very smart dd, in a way. Is
> that possible?
>
> Finally, I heard mention (fromo Peter Breuer I think) of a raid5 patch
> that tolerates sector read errors and re-writes automagically. Any info
> on that would be interesting.
>
> Thanks for your time
> -Mike
> -
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^ permalink raw reply [flat|nested] 5+ messages in thread