Re: [RFC PATCH] dm-csum: A new device mapper target that checks data integrity

[Goswin von Brederlow <goswin-v-b@web.de>]

Alberto Bertogli <albertito@blitiri.com.ar> writes:

> On Sun, Jun 28, 2009 at 10:34:17AM +1000, Neil Brown wrote:
>> On Tuesday May 26, albertito@blitiri.com.ar wrote:
>> > On Tue, May 26, 2009 at 12:33:01PM +0200, Goswin von Brederlow wrote:
>> > > > This scheme assumes writes to a single sector are atomic in the presence of
>> > > > normal crashes, which I'm not sure if it's something sane to assume in
>> > > > practise. If it's not, then the scheme can be modified to cope with that.
>> > > 
>> > > What happens if you have multiple writes to the same sector? (assuming
>> > > you ment "before" above)
>> > > 
>> > > - user writes to sector
>> > > - queue up write for M1 and data1
>> > > - M1 writes
>> > > - user writes to sector
>> > > - queue up writes for M2 and data2
>> > > - data1 is thrown away as data2 overwrites it
>> > > - M2 writes
>> > > - system crashes
>> > > 
>> > > Now both M1 and M2 have a different checksum than the old data left on
>> > > disk.
>> > > 
>> > > Can this happen?
>> > 
>> > No, parallel writes that affect the same metadata sectors will not be allowed.
>> > At the moment there is a rough lock which does not allow simultaneous updates
>> > at all, I plan to make that more fine-grained in the future.
>> 
>> Can I suggest a variation on the above which, I think, can cause a
>> problem.
>> 
>>  - user writes data-A' to sector-A (which currently contains data-A)
>>  - queue up write for M1 and data-A'
>>  - M1 is written correctly.
>>  - power fails (before data-A' is written)
>> reboot
>>  - read sector-A, find data-A which matches checksum on M2, so
>>    success.
>> 
>> So everything is working perfectly so far...
>> 
>>  - write sector-B (in same 62-sector range as sector-A).
>>  - queue up write for M2 and data-B
>>  - those writes complete
>>  - read sector-A.  find data-A, which doesn't match M1 (that has
>>    data-A') and doesn't match M2 (which is mostly a copy of M1),
>>    so the read fails.
>
> The thing is that M2 is not a copy of M1. When updating M2 for data-B, the
> procedure is not "copy M1, update sector-B's checksum, write" but "read M2,
> update sector-B's checksum, write". So as long as there are no writes to
> sector-A, M1 will have the incorrect checksum and M2 will have the correct
> one, regardless of writes to the other sectors.
>
> However, a troubling scenario based on yours could be:
>
>  - M2 has the right checksum but is older, M1 has the wrong checksum but is
>    newer.
>  - user writes data-A'' to sector'A
>  - queue up write for M2 (chosen because it is older)
>  - M2 is written correctly
>  - power fails before data-A'' is written
>
> At that point, data-A is written at sector-A, but both M1 and M2 have
> incorrect checksums for it.
>
> I'll try to come up with a better scheme that copes with this kind of
> scenarios and post an updated patch.
>
> Thanks a lot,
> 		Alberto

When the newer block has the wrong checksum you first need to correct
that. If you find a wrong checksum on read that is easy to do. But you
won't detect this on writes.

One solution I can think of is this:

- user writes to sector A
- compare checksum of sector A in M1 and M2
  if checksums differ:
  - read sector A and calculate checksum
  - if M1 has the right checksum update M2
  - wait
- write new checksum to M1
- wait
- write data to sector A
- wait
- write new checksum to M2

MfG
        Goswin