2012-08-01 16:22, Sašo Kiselkov пишет:
On 08/01/2012 12:04 PM, Jim Klimov wrote:
Probably DDT is also stored with 2 or 3 copies of each block,
since it is metadata. It was not in the last ZFS on-disk spec
from 2006 that I found, for some apparent reason ;)



The idea of the pun was that the latest available full spec is
over half a decade old, alas. At least I failed to find any one
newer, when I searched last winter. And back in 2006 there was
no dedup nor any mention of it in the spec (surprising, huh? ;)

Hopefully with all the upcoming changes - including integration
of feature flags and new checksum and compression algorithms,
the consistent textual document of "Current ZFS On-Disk spec in
illumos(/FreeBSD/...)" would appear and be maintained up-to-date.


That's probably because it's extremely big (dozens, hundreds or even
thousands of GB).

Availability of the DDT is IMHO crucial to a deduped pool, so
I won't be surprised to see it forced to triple copies. Not
that it is very difficult to check with ZDB, though finding
the DDT "dataset" for inspection (when I last tried) was not
an obvious task.

//Jim
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