Peter Rabbitson wrote:
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.
I totally agree, not doing the most likely to be correct thing seems to
be the one argument for hardware raid. There are two case in which
software can determine (a) if it is likely that there is a single bad
block, and (b) what the correct value for that block is.
raid1 - more than one copy
If there are multiple copies of the data, and N-1 agree, then it is
more likely that the mismatched copy is the bad one, and should be
rewritten with the data in the other copies. This is never less likely
to be correct than selecting one copy at random and writing it over all
others, so it can only be a help.
raid6 - assume and check
Given an error in raid6, if the parity A appears correct and the
parity B does not, assume that the non-matching parity is bad and
regenerate.
If neither parity appears correct, for each data block assume it is
bad and recalculate a recovery value using A nd B parities. If the data
pattern generated is the same for recovery using either parity, assume
that the data is bad and rewrite.
Again, this is more likely to be correct than assuming that both
parities are wrong. Obviously if no "most likely" bad data or parity
information can be identified then recalculating both parity blocks is
the only way to "fix" the array, but it leaves undetectable bad data. I
would like an option to do repairs using these two methods, which would
give a high probability that whatever "fixes" were applied were actually
recovering the correct data.
Yes, I know that errors like this are less common than pure hardware
errors, does that justify something less than best practice during recovery?
--
Bill Davidsen <davidsen@xxxxxxx>
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark
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