From: Sage Weil <sweil@redhat.com>
To: Li Wang <liwang@ubuntukylin.com>
Cc: Samuel Just <sjust@redhat.com>, Josh Durgin <jdurgin@redhat.com>,
ceph-devel <ceph-devel@vger.kernel.org>
Subject: Re: [RFC] Implement a new journal mode
Date: Fri, 29 May 2015 08:46:42 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.00.1505290840180.6462@cobra.newdream.net> (raw)
In-Reply-To: <55683011.7000307@ubuntukylin.com>
On Fri, 29 May 2015, Li Wang wrote:
> An important usage of Ceph is to integrate with cloud computing platform
> to provide the storage for VM images and instances. In such scenario,
> qemu maps RBD as virtual block devices, i.e., disks to a VM, and
> the guest operating system will format the disks and create file
> systems on them. In this case, RBD mostly resembles a 'dumb' disk. In
> other words, it is enough for RBD to implement exactly the semantics of
> a disk controller driver. Typically, the disk controller itself does
> not provide a transactional mechanism to ensure a write operation done
> atomically. Instead, it is up to the file system, who manages the disk,
> to adopt some techniques such as journaling to prevent inconsistency,
> if necessary. Consequently, RBD does not need to provide the
> atomic mechanism to ensure a data write operation done atomically,
> since the guest file system will guarantee that its write operations to
> RBD will remain consistent by using journaling if needed. Another
> scenario is for the cache tiering, while cache pool has already
> provided the durability, when dirty objects are written back, they
> theoretically need not go through the journaling process of base pool,
> since the flusher could replay the write operation. These motivate us
> to implement a new journal mode, metadata-only journal mode, which
> resembles the data=ordered journal mode in ext4. With such journal mode
> is on, object data are written directly to their ultimate location,
> when data written finished, metadata are written into the journal, then
> the write returns to caller. This will avoid the double-write penalty
> of object data due to the WRITE-AHEAD-LOGGING, potentially greatly
> improve the RBD and cache tiering performance.
>
> The algorithm is straightforward, as before, the master send
> transaction to slave, then they extract the object data write
> operations and apply them to objects directly, next they write the
> remaining part of the transaction into journal, then slave ack master,
> master ack client. For some special operations such as 'clone', they
> can be processed as before by throwing the entire transaction into
> journal, which makes this approach an absolutely-better optimization
> in terms of performance.
>
> In terms of consistency, metadata consistency is ensured, and
> the data consistency of CREATE and APPEND are also ensured, just for
> OVERWRITE, it relies on the caller, i.e., guest file system for RBD,
> cache flusher for cache tiering to ensure the consistency. In addition,
> there remains a problem to be discussed that how to interact with the
> scrub process while the object data consistency may not ensured now.
Right. This is appealing from a performance perspective, but I'm worried
it will throw out too many other assumptions in RADOS that will cause
pain. The big one is that RADOS will no longer know if the version on the
object metadata matches the data. This will be most noticeable from
scrub, which will have no idea whether the inconsistency is from a partial
write or from a disk error. And when that happens, it would have to guess
which object is the right one--a guess that can easily be wrong if there
is rebalancing or recovery that may replicate the partially updated
object.
Maybe we can journal metadata before applying the write to indicate the
object is 'unstable' (undergoing an overwrite) to help out?
I'm not sure. Honestly, I would be more interested in investing our time
in making the new OSD backends handle overwrite more efficiently, by
avoiding write-ahead in the easy cases (append, create) as newstore
does, and/or by doing some sort of COW when we do overwrite, or some other
magic that does an atomic swap-data-into-position (e.g., by abusing the
xfs defrag ioctl).
What do you think?
sage
>
> We are actively working on it and have done part of the implementation,
> want to hear the feedback of the community, and we may submit it as a
> blueprint to under discussion in coming CDS.
>
> Cheers,
> Li Wang
>
>
next prev parent reply other threads:[~2015-05-29 15:46 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-29 9:23 [RFC] Implement a new journal mode Li Wang
2015-05-29 15:46 ` Sage Weil [this message]
2015-06-02 9:28 ` Li Wang
2015-06-02 10:55 ` Haomai Wang
2015-06-03 3:42 ` Li Wang
2015-06-02 15:17 ` Sage Weil
2015-06-18 13:34 ` Li Wang
2015-06-18 14:14 ` Sage Weil
2015-06-19 3:45 ` Li Wang
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