Ceph

h1. Rados - metadata-only journal mode

*Summary*

This is for metadata-only journal mode. In this mode, for write operation,

the OSD will journal only metadata, without journaling the written data.

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. 

*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.

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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*

1  Submit transaction A into journal, mark a <offset, length>

non-journaling data write in pglog (peering) or omap/xattrs(scrub)

2 Write data to object

3 Submit transaction B into journal, to update the metadata as well as

pglog as usual 

As long as one OSD in the PG has succeeded, the PG will be recovered to 

a consistent and correct state by peering; The only potentially problematical 

situation is the PG down as a whole, and none of the OSDs has finished Step (3), 

and at least one of the OSDs has finished Step (1). In that 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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