[dm-devel] Some thoughts about providing data block checksumming for ext4
mpatocka at redhat.com
Wed Nov 5 00:27:54 UTC 2014
On Tue, 4 Nov 2014, Mikulas Patocka wrote:
> > > Recovery after a power fail
> > > ---------------------------
> > >
> > > If the dm-protected device was not cleanly shut down, then we need to
> > > examine all of the checksum blocks in the Active Area. For each
> > > checksum block in the AA, the checksums for all of their data blocks
> > > should machine either the checksum found in the AA, or the checksum
> > > found in the checksum block in the checksum group.
> > ... and if the checksum of the block matches BOTH the checksum in the AA
> > and the checksum in the checksum group (because of checksum function
> > collision), you don't know which 4-bit nibble belongs to the data in the
> > block.
> Though, I realize that you could avoid this problem by selecting the
> appropriate checksum function - that never results in collision if the
> 4-bit nibble differs.
Hmm, that is still not sufficient.
Suppose that "a" and "b" is sector content without the 4-bit nibble and
"x" and "y" are two different nibbles.
Now, we have this situation:
a + x -> checksum1
b + x -> checksum1
a + y -> checksum2
b + y -> checksum2
Suppose that we do crash recovery and we have (x,checksum1) in the
checksum block and (y,checksum2) in the active area - we can't really tell
which one is valid.
So you really need cryptographic hashes instead of checksums to avoid the
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