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Li Wang, 06/30/2015 09:42 AM

Rados - metadata-only journal mode

Currently the Ceph community is thinking of eliminating the double write
penalty of write ahead logging, newstore is a great design which implements
create, append operations in an copy on read way, while maintaining all
the original semantics. This makes newstore a general purpose optimization,
especially suitable for the write once scenarios. Metadata-only journal mode
intends to do in a more aggressive way, that is, not journal object data at all.
This applies to two major kinds of situations, one is that the atomicity for
object data modification may not need, for example, RBD to simulate a disk
in cloud platform. The second is those double journaling situations, for example,
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 always replay the write operation.
Metadata-only journal mode, to some extent, 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. It guarantees the consistency in terms of RADOS name space, and the data
consistency among object copies. However, the object data may not be correct.
Later we will demonstrate that this rarely happens.


Li Wang ()
Yunchuan Wen ()

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Current Status
Please describe the current status of Ceph as it relates to this blueprint. Is there something that this replaces? Are there current features that are related?

Detailed Description
The algorithm is as follows,

1 Submit transaction A into journal, add a record for <offset, length>
non-journaling data write in omap,
2 Write data to object
3 Submit transaction B into journal, to update the metadata as well as
pglog as usual, and revert the operations of transaction A

As long as one osd in the pg has succeeded, the pg will be recovered to
a consistent and correct state by peering; If the PG down as a whole,
there are the following situations,
(1) None of the osds finishes step 1, nothing happen;
(2) At least one of the osds finishes step 3, journaling and
peering will recover the pg to a consistent and correct state;
(3) none of the osds has finished step (3), and at least one of the osds has
finished step (1), this is the only potentially problematical situation,
in this case, we revise peering or scrub to make them realize the
semantics of transaction A, and randomly choose one osd to synchronize its
content of written area to other copies. We prefer to leave it the scrub's job.
Since scrub is done asynchronously, and maybe be scheduled to run late, during
this period, client's resend may have recovered the content to consistent.

Work items
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Build / release tasks
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Documentation tasks
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Deprecation tasks
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