RAID 1 or Hammer

Matthew Dillon dillon at
Mon Jan 12 19:06:51 PST 2009

:But Hammer won't notify this as well, right? IIRC, Hammer does CRC only
:on meta-data.
:   Michael

    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.

					Matthew Dillon 
					<dillon at>

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