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Li Wang, 06/10/2015 11:35 AM


Rados - metadata-only journal mode

Summary
This is for metadata-only journal mode. 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 ()
Yunchuan Wen ()
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Current Status
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Detailed Description
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Work items
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