A device dedicated for metadata?

[wks1986 <wks1986@gmail.com>]

Hello

Recently I am learning about the Btrfs.

My requirement is to construct a cross-device btrfs volume consisting
of a single SSD and many (much larger) HDDs.  The tricky part is that
I want the SSD to be dedicated to metadata since SSDs are much faster
(and more expensive) than HDDs and metadata are much more frequently
queried than data.  And I do not want ordinary data to use up the
precious space on the SSD in case there would be no room for metadata.

I have figured out that I can keep the metadata stored inside the
first device by applying the "-m single" option to mkfs.btrfs.
However, I do not know how to require that data should NOT be
allocated on the SSD.  I read the mkfs code.  It appears to me that it
can be worked around by excluding the SSD from the "raid group" for
ordinary data.

A recent patch have been proposed to move "hot" (according to
statistics) data to SSDs in order to improve performance.  My case is
similar, but I know in advance that metadata are hotter.

I am not very confident about what I have found, so I need some
comment.  Is it necessary to modify the Btrfs code?  On the other
hand, I have not figured out exactly how much metadata there would be
with a given amount of data.  How significant will it be to exclude
ordinary data from the SSD?

Kunshan Wang