Duncan wrote on 2016/01/12 04:13 +0000:
Qu Wenruo posted on Tue, 12 Jan 2016 11:09:23 +0800 as excerpted:
Now we hope to add support to enable/disable dedup per-file.
Much like current NODATACOW/NOCOMPRESS for inode.
How is this going to work?
NODATACOW/NOCOMPRESS can apply to a single file. But a dup flag, by
definition, needs two files, except for the special case of parts of a
file duplicating other parts of the same file. Is there going to be some
background thread that checks for dups and reflinks duplicated extents if
both files have the dup flag set? What if one has it on and one has it
off?
You are still thinking in the way off-band dedup.
For off-band dedup, we need two extents to compare.
But for in-band dedup, we are not using reflink or similar facility.
Instead, we have a hash pool, recording part or all of our known hashes
of extents.
So the things should be quite easy to understand:
For normal case (no NODEDUP flag), valid data(page cache) will be hashed
to find if it's a duplicated one.
For NODEDUP flag case, all its page cache just direct write to disk or
compressed then write to disk.
No hash will be calculated.
Thanks,
Qu
Presumably, if a file has it on and it is copied (so a new file), the
copy would be reflinked. But if the flag is off, does that make the file
actually data-copy, by default, even if cp decides to do a reflink copy
by default? And does the copy automatically have the dup flag set as
well, or does the original instance set dup, while the new copy, reflinked
to the old one due to that dup flag, still have the dup flag unset, until
the user sets it?
OTOH, I can see such an attribute for dirs making more sense, since it
could be inherited much like the NOCOW attribute, and new files created
there could automatically be checked against the current files to see if
parts are dup, and reflink them if so.
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