Re: Triple parity and beyond

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

Hi David,

On Thu, Nov 21, 2013 at 09:31:46PM +0100, David Brown wrote:
[...]
> If this can all be done to give the user an informed choice, then it
> sounds good.

that would be my target.
To _offer_ more options to the (advanced) user.
It _must_ always be under user control.

> One issue here is whether the check should be done with the filesystem
> mounted and in use, or only off-line.  If it is off-line then it will
> mean a long down-time while the array is checked - but if it is online,
> then there is the risk of confusing the filesystem and caches by
> changing the data.

Currently, "raid6check" can work with FS mounted.
I got the suggestion from Neil (of course).
It is possible to lock one stripe and check it.
This should be, at any given time, consistent
(that is, the parity should always match the data).
If an error is found, it is reported.
Again, the user can decide to fix it or not,
considering all the FS consequences and so on.

> Most disk errors /are/ detectable, and are reported by the underlying
> hardware - small surface errors are corrected by the disk's own error
> checking and correcting mechanisms, and larger errors are usually
> detected.  It is (or should be!) very rare that a read error goes
> undetected without there being a major problem with the disk controller.
>  And if the error is detected, then the normal raid processing kicks in
> as there is no doubt about which block has problems.

That's clear. That case is an "erasure" (I think)
and it is perfectly in line with the usual operation.
I'm not trying to replace this mechanism.
 
> If you can be /sure/ about which data block is incorrect, then I agree -
> but you can't be /entirely/ sure.  But I agree that you can make a good
> enough guess to recommend a fix to the user - as long as it is not
> automatic.

One typical case is when many errors are
found, belonging to the same disk.
This case clearly shows the disk is to be
replaced or the interface checked...
But, again, the user is the master, not the
machine... :-)
 
> For most ECC schemes, you know that all your blocks are set
> synchronously - so any block that does not fit in, is an error.  With
> raid, it could also be that a stripe is only partly written - you can

Could it be?
I would consider this an error.
The stripe must always be consistent, there
should be a transactional mechanism to make
sure that, if read back, the data is always
matching the parity.
When I write "read back" I mean from whatever
the data is: physical disk or cache.
Otherwise, the check must run exclusively on
the array (no mounted FS, no other things
running on it).

> have two different valid sets of data mixed to give an inconsistent
> stripe, without any good way of telling what consistent data is the best
> choice.
>  
> Perhaps a checking tool can take advantage of a write-intent bitmap (if
> there is one) so that it knows if an inconsistent stripe is partly
> updated or the result of a disk error.

Of course, this is an option, which should be
taken into consideration.

Any improvement idea is welcome!!!

bye,

-- 

piergiorgio