On 6/15/2010 2:59 AM, Darren J Moffat wrote:
On 15/06/2010 10:48, Erik Trimble wrote:
Note, that if the checksum type used is different in on host A vs host
B, you can't do any form of extra dedup this way. I'd have to check if
different compression types would cause problems, as I can't recall if
that affects the actual checksum being stored (I think it does, as I'm
pretty sure ZFS stores the post-compressed block checksum, but I'm not
100% sure). All of these problems would easily be detectable, and a
properly written application would be able to report back to the user
such conditions (and should then fall back to the standard 'zfs send -D'
behavior).
Both the checksum and compression are used in the key for the DDT.
On disk the checksum is of the compressed block, in the send stream is
is the uncompressed block (since zfs send is above the DMU layer).
So, that would be even better. It would allow for "super" dedup even if
the compression algorithms were different.
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
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