Hi Sage,
thanks for your quick response. Javen and I once the zfs developer,are
currently focusing on how to
leverage some of the zfs ideas to improve the ceph backend performance
in userspace.
Based on your encouraging reply, we come up with 2 schemes to continue
our future work
1. the scheme one: using the entire new FS to replace rocksdb+bluefs,
the FS itself handles the mapping of
oid->fs-object(kind of zfs dnode) and the according attrs used by ceph.
Despite the implemention challenges you mentioned about the in-order
enumeration of objects during backfill, scrub, etc (the
same situation we also confronted in zfs, the ZAP features help us
a lot).
From performance or architecture point of view, it looks more clear
and clean, would you suggest us to give a try ?
2. the scheme two: As your last suspect, we just temporarily implemented
the simple version of the FS
which leverage libzpool ideas to plug into rocksdb underneath as
your bluefs did
precious your insightful reply.
Thanks
On 2016年01月07日 21:19, Sage Weil wrote:
On Thu, 7 Jan 2016, Javen Wu wrote:
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.
BlueStore
=========
RocksDB
+-----------+ +-----------+
| onode | | |
| WAL | | |
| omap | | |
+-----------+ | bdev |
| | | |
| XFS | | |
| | | |
+-----------+ +-----------+
This is the picture before BlueFS enters the picture.
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.
Right. BlueFS is a really simple "file system" that is *just* complicated
enough to implement the rocksdb::Env interface, which is what rocksdb
needs to store its log and sst files. The after picture looks like
+--------------------+
| bluestore |
+----------+ |
| rocksdb | |
+----------+ |
| bluefs | |
+----------+---------+
| block device |
+--------------------+
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.
I think it makes a lot of sense, but there are a few challenges. One
reason we use rocksdb (or a similar kv store) is that we need in-order
enumeration of objects in order to do collection listing (needed for
backfill, scrub, and omap). You'll need something similar on top of zfs.
I suspect the simplest path would be to also implement the rocksdb::Env
interface on top of the zfs libraries. See BlueRocksEnv.{cc,h} to see the
interface that has to be implemented...
sage
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