DragonFly users List (threaded) for 2009-01
Re: RAID 1 or Hammer
:But Hammer won't notify this as well, right? IIRC, Hammer does CRC only
HAMMER CRC's both data and meta-data. The meta-data is self-CRC'd,
however, which is one step down from what ZFS does. ZFS uses
recursive check data, creating an unbroken integrity chain to the
root of the filesystem. A self-CRC can result in a delusion in that
if a new block is not written the old block still contains a valid
self-CRC of the old information.
At the moment the DATA CRC is not always checked during normal filesystem
operation due to the direct-I/O nature of the buffer cache. It is
checked when HAMMER does a reblocking operation.
HAMMER's CRC's are only 32 bits... not sufficient for the kinds of
soft-raid tasks ZFS uses its much larger check data for.
I have some ideas on how to give HAMMER a recursive integrity check.
It is expensive to do (as is ZFS's recursive integrity check) but I
can definitely see some people wanting such a feature.