Use fast device only for metadata?

[Nikolaus Rath <Nikolaus@rath.org>]

Hello,

I have a large home directory on a spinning disk that I regularly
synchronize between different computers using unison. That takes ages,
even though the amount of changed files is typically small. I suspect
most if the time is spend walking through the file system and checking
mtimes.

So I was wondering if I could possibly speed-up this operation by
storing all btrfs metadata on a fast, SSD drive. It seems that
mkfs.btrfs allows me to put the metadata in raid1 or dup mode, and the
file contents in single mode. However, I could not find a way to tell
btrfs to use a device *only* for metadata. Is there a way to do that?

Also, what is the difference between using "dup" and "raid1" for the
metadata? 

Best,
-Nikolaus

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