[Discuss] SSD drives vs. Mechanical drives

[Richard Pieri]

Bill Bogstad wrote:
> ECC is not 100%.

It's not intended to be 100%. It's intended to fault when cosmic ray
strikes causes random bit flips.


> Nor does it protect against transient CPU/memory
> cache errors during
> checksum computation.  If you are saying that ZFS can then I will happily read

You have a block of data in memory, and you calculate a checksum, and
write the data and checksum out, and the controller says that the writes
are completed, and you read that checksum and data back and calculate a
checksum on the data that was read. Then compare the checksums. If they
match? What's on disk matches what's in RAM. If they don't then there's
a problem somewhere and the file system driver knows it. There's more to
it. ZFS and Btrfs don't just checksum blocks and extents. They checksum
the checksums. There's a checksum hierarchy from the root on down
(Merkle tree).

Assuming that you have ECC RAM and disks that honor sync commands then
yes, ZFS and Btrfs are doing some kind of awesome and you really should
read the whitepapers about them.

-- 
Rich P.