Hi Sage,

Sorry to bother you. I am not sure if it is appropriate to send email to you
directly, but I cannot find any useful information to address my confusion
from Internet. Hope you can help me.

Occasionally, I heard that you are going to start BlueFS to eliminate the
redudancy between XFS journal and RocksDB WAL. I am a little confused.
Is the Bluefs only to host RocksDB for BlueStore or it's an
alternative of BlueStore?

I am a new comer to CEPH, I am not sure my understanding is correct about
BlueStore. BlueStore in my mind is as below.

+-----------+          +-----------+
|   onode   |          |           |
|    WAL    |          |           |
|   omap    |          |           |
+-----------+          |   bdev    |
|           |          |           |
|   XFS     |          |           |
|           |          |           |
+-----------+          +-----------+

I am curious if BlueFS is able to host RocksDB, actually it's already a
"filesystem" which have to maintain blockmap kind of metadata by its own
WITHOUT the help of RocksDB. When BlueFS is introduced into the picture,
why RocksDB is needed yet? So I guess BlueFS is an alternative of BlueStore
and it's a new ObjectStore without leveraging RocksDB.

Is my understanding correct?

The reason we care the intention and the design target of BlueFS is that I had
discussion with my partner Peng.Hse about an idea to introduce a new
ObjectStore using ZFS library. I know CEPH supports ZFS as FileStore backend already, but we had a different immature idea to use libzpool to implement a new ObjectStore for CEPH totally in userspace without SPL and ZOL kernel module. So that we can align CEPH transaction and zfs transaction in order to avoid
double write for CEPH journal.
ZFS core part libzpool (DMU, metaslab etc) offers a dnode object store and
it's platform kernel/user independent. Another benefit for the idea is we
can extend our metadata without bothering any DBStore.

Frankly, we are not sure if our idea is realistic so far, but when I heard of
BlueFS, I think we need to know the BlueFS design goal.

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