On Sun, Jun 28, 2009 at 10:34:17AM +1000, Neil Brown wrote:

On Tuesday May 26, albertito@blitiri.com.ar wrote:

quoted

On Tue, May 26, 2009 at 12:33:01PM +0200, Goswin von Brederlow wrote:

quoted

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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