On Sat, Jan 16, 2016 at 9:10 AM, Duncan [off-list ref] wrote:

Al posted on Sat, 16 Jan 2016 12:27:16 +0000 as excerpted:

quoted

Is there any urgency for dedup? What's wrong with storing the hash on
disk with the block and having a separate process dedup the written data
over time;

There's actually uses for both inline and out-of-line[1] aka delayed
dedup.  Btrfs already has a number of independent products doing various
forms of out-of-line dedup, so what's missing and being developed now is
the inline dedup option, which being directly in the write processing,
must be handled by btrfs itself -- it can't be primarily done by third
parties with just a few kernel calls, like out-of-line dedup can.

Does the out-of-line dedup option actually utilize stored hashes, or
is it forced to re-read all the data to compute hashes?  If it is
collecting checksums/etc is this done efficiently?

I think he is actually suggesting a hybrid approach where a bit of
effort is done during operations to greatly streamline out-of-line
deduplication.  I'm not sure how close we are to that already, or if
any room for improvement remains.

--
Rich