Andrey Kuzmin wrote:
On Tue, Apr 28, 2009 at 2:02 PM, Chris Mason <chris.ma...@oracle.com> wrote:
On Tue, 2009-04-28 at 07:22 +0200, Thomas Glanzmann wrote:
Hello Chris,
There is a btrfs ioctl to clone individual files, and this could be used
to implement an online dedup. But, since it is happening from userland,
you can't lock out all of the other users of a given file.
So, the dedup application would be responsible for making sure a given
file was not being changed while the dedup scan was running.
I see, does that mean that I can not do ,,dedup'' for files that are
currently opened by a userland program?
No, but it does mean the dedup done from userland is racey. Picture
Race disappears if (background) dedupe is run against snapshot(s).
only if the snapshots are made read-only.
btrfs has writable snapshots.
jim
Regards,
Andrey
this:
process A:
create some_file # some_file matches the contents of another file
dedup proc:
check some_file
decide to start block dedup
process A:
modify some_file
dedup proc:
progress through block dedup
So, this will happily replace blocks in some_file with the dedup blocks.
But there's no way to atomically swap them.
We could create new ioctls for this, basically a variant of the clone
file ioctl that makes sure a given set of pages has a given sum (or
strict memory contents) before doing the swap.
But they don't exist yet.
-chris
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