Ceph

h1. Rados - metadata-only journal mode

*Summary*

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.

*Owners*

Li Wang (liwang@ubuntukylin.com)

Yunchuan Wen (yunchuanwen@ubuntukylin.com)

Name

*Interested Parties*

If you are interested in contributing to this blueprint, or want to be a "speaker" during the Summit session, list your name here.

Name (Affiliation)

Name (Affiliation)

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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*

This section should contain a list of work tasks created by this blueprint.  Please include engineering tasks as well as related build/release and documentation work.  If this blueprint requires cleanup of deprecated features, please list those tasks as well.

*Coding tasks*

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*Build / release tasks*

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*Documentation tasks*

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*Deprecation tasks*

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