Re: Redundancy check using "echo check > sync_action": error reporting?

[Peter Rabbitson <rabbit+list@rabbit.us>]

Theodore Tso wrote:
> On Thu, Mar 20, 2008 at 03:19:08PM +0100, Bas van Schaik wrote:
>>> There's no explicit message produced by the md module, no.  You need to
>>> check the /sys/block/md{X}/md/mismatch_cnt entry to find out how many
>>> mismatches there are.  Similarly, following a repair this will indicate
>>> how many mismatches it thinks have been fixed (by updating the parity
>>> block to match the data blocks).
>>>   
>> Marvellous! I naively assumed that the module would warn me, but that's
>> not true. Wouldn't it be appropriate to print a message to dmesg if such
>> a mismatch occurs during a check? Such a mismatch clearly means that
>> there is something wrong with your hardware lying beneath md, doesn't it?
> 
> If a mismatch is detected in a RAID-6 configuration, it should be
> possible to figure out what should be fixed (since with two hot spares
> there should be enough redundancy not only to detect an error, but to
> correct it.)  Out of curiosity, does md do this automatically, either
> when reading from a stripe, or during a resync operation?
> 

In my modest experience with root/high performance spool on various raid 
levels I can pretty much conclude that the current check mechanism doesn't do 
enough to give power to the user. We can debate all we want about what the MD 
driver should do when it finds a mismatch, yet there is no way for the user to 
figure out what the mismatch is and take appropriate action. This does not 
apply only to RIAD5/6 - what about RAID1/10 with >2 chunk copies? What if the 
only wrong value is taken and written all over the other good blocks?

I think that the solution is rather simple, and I would contribute a patch if 
I had any C experience. The current check mechanism remains the same - 
mismatch_cnt is incremented/reset just the same as before. However on every 
mismatching chunk the system printks the following:

1) the start offset of the chunk(md1/10) or stripe(md5/6) within the MD device
2) one line for every active disk containing:
	a) the offset of the chunk within the MD componnent
	b) a {md5|sha1}sum of the chunk

In a common case array this will take no more than 8 lines in dmesg. However 
it will allow:

1) For a human to determine at a glance which disk holds a mismatching chunk 
in raid 1/10
2) Determine the same for raid 6 using a userspace tool which will calculate 
the parity for every possible permutation of chunks
3) using some external tools to determine which file might have been affected 
on the layered file system


Now of course the problem remains how to repair the array using the 
information obtained above. I think the best way would be to extend the syntax 
of repair itself, so that:

echo repair > .../sync_action would use the old heuristics

echo repair <mdoffset> <component N> > .../sync_action will update the chunk 
on drive N which corresponds to the chunk/stripe at mdoffset within the MD 
device, using the information from the other drives, and not the other way 
around as might happen with just a repair.

Just my 2c

Peter