HAMMER de-dup in master
Ok folks, Ilya's HAMMER de-dup is now in master, but before you get too
excited please note the following:
* The hammer filesystem must be upgraded to version 5 (from 4). This
doesn't actually change anything but 2.8 doesn't have the upgrade
code yet so if you upgrade it in 2.9, 2.8 will refuse to mount the
filesystem.
This is on purpose for now. In a week or two I'll MFC the upgrade
to 2.8.
* de-dup is highly experimental. There is a known issue where reblocking
can sometimes undo some of the de-dupping that was done.
* de-dup is highly experimental, don't use it on a production server yet.
* de-dupped data tends to be more fragmented (this is just a general
thing that is true for de-dupping, no matter the filesystem). This
means you are trading off a little performance for the de-dupped
blocks.
It should only be used when disk space is a concern, and only if it
would effective. The 'hammer dedup-simulate' can be used to see
how well de-dup might perform without actually doing it (this will run
on 2.8 too).
* Did I mention that de-dup is highly experimental? It needs a lot
more testing before I would consider it production-ready.
-Matt
Matthew Dillon
<dillon@backplane.com>
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