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

[Alberto Bertogli]

On Tue, May 26, 2009 at 12:33:01PM +0200, Goswin von Brederlow wrote:
> Alberto Bertogli <albertito at blitiri.com.ar> writes:
> > On Mon, May 25, 2009 at 02:22:23PM +0200, Goswin von Brederlow wrote:
> >> Alberto Bertogli <albertito at blitiri.com.ar> writes:
> >> > I'm writing this device mapper target that stores checksums on writes and
> >> > verifies them on reads.
> >> 
> >> How does that behave on crashes? Will checksums be out of sync with data?
> >> Will pending blocks recalculate their checksum?
> >
> >    To guarantee consistency, two imd sectors (named M1 and M2) are kept for
> >    every 62 data sectors, and the following procedure is used to update them
> >    when a write to a given sector is required:
> >
> >     - Read both M1 and M2.
> >     - Find out (using information stored in their headers) which one is newer.
> >       Let's assume M1 is newer than M2.
> >     - Update the M2 buffer to mark it's newer, and update the new data's CRC.
> >     - Submit the write to M2, and then the write to the data, using a barrier
> >       to make sure the metadata is updated _after_ the data.
> 
> Consider that the disk writes the data and then the system
> crashes. Now you have the old checksum but the new data. The checksum
> is out of sync.
> 
> Don't you mean that M2 is written _before_ the data? That way you have
> the old checksum in M1 and the new in M2. One of them will match
> depending on wether the data gets written before a crash or not. That
> would be more consistent with your read operation below.

Yes, the comment is wrong, thanks for noticing. That is how it's implemented.


> >    Accordingly, the read operations are handled as follows:
> >
> >     - Read both the data, M1 and M2.
> >     - Find out which one is newer. Let's assume M1 is newer than M2.
> >     - Calculate the data's CRC, and compare it to the one found in M1. If they
> >       match, the reading is successful. If not, compare it to the one found in
> >       M2. If they match, the reading is successful; otherwise, fail. If
> >       the read involves multiple sectors, it is possible that some of the
> >       correct CRCs are in M1 and some in M2.
> >
> >
> > The barrier will be (it's not done yet) replaced with serialized writes for
> > cases where the underlying block device does not support them, or when the
> > integrity metadata resides on a different block device than the data.
> >
> >
> > 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.

Note that bios that affect more than one sector are just fine, since the
metadata sector will be properly updated.

Thanks,
		Alberto