Performance Considerations Write transactions are very fast since they only involve writing the content once versus twice for rollback-journal transactions and because the writes are all sequential. In Ceph luminous this store is the default when creating OSDs.
For example, customers are adapting OLTP workloads to run on Ceph when they migrate from traditional enterprise storage solutions. Information previously stored in extended attributes is now stored in RocksDB and that relatively tiny database has its own WAL write ahead log. You can see this disparity for yourself by comparing the ceph-osd process RSS with the output from ceph daemon osd.
The only way we have found to guarantee that all processes accessing the same database file use the same shared memory is to create the shared memory by mmapping a file in the same directory as the database itself.
We have taken control of a larger portion of the storage stack—all the way down to the raw block device—providing greater control of the IO flow and data layout. The opening process must have write privileges for "-shm" wal-index shared memory file associated with the database, if that file exists, or else write access on the directory containing the database file if the "-shm" file does not exist.
The downside to this configuration is that transactions are no longer durable and might rollback following a power failure or hard reset. However, compile-time and run-time options exist that can disable or defer this automatic checkpoint.
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There are interesting developments in Ceph Jewel, namely the bluestore backend which may change that. In BlueStore, the internal journaling needed for consistency is much lighter-weight, usually behaving like a metadata journal and only journaling small writes when it is faster or necessary to do so.
This avoids any intervening layers of abstraction such as local file systems like XFS that may limit performance or add complexity. I spent virtually all of Thursday identifying the problem and trying to find of a solution.
XFS was developed for Silicon Graphics, and is a mature and stable filesystem.Another way to think about the difference between rollback and write-ahead log is that in the rollback-journal approach, there are two primitive operations, reading and writing, whereas with a write-ahead log there are now three primitive operations: reading, writing, and checkpointing.
Aug 23, · Ceph BlueStore - Not always faster than FileStore. (write ahead log). The WAL is automatically located together with the DB, but can be split out, should the system additionally have NVMe or 3D X-Point memory.
Recreate OSDs using FileStore, with journal on SSD partition (using partition UUIDs, Proxmox appears to default to device name). The file store flusher forces data from large write operations to be written out using the sync file range option before the synchronization in order to reduce the cost of the eventual synchronization.
In practice, disabling the file store flusher seems to improve performance in some cases. Challenges in Using Persistent Memory In Distributed Storage Systems Dan Lambright HPC, sequential, random, mixed read/write/transfer size, etc Ceph Bluestore Write ahead log DM cache XFS journal Improve Parts of System With SCM Heterogeneous storage.
Bluestore: A new storage engine for Ceph Allen Samuels, Engineering Fellow March 4, Write Ahead Log XFS KeyValueDB OSD. Ceph Deployment Options •Ceph Journal on Flash 3X Replication RadosGW Write Tests Filestore KB Chunks Filestore 4MB Chunks Bluestore KB Chunks Bluestore 4MB Chunks.
The benefits of Bluestore are a direct to block OSD, without filesystem overhead or the need for a “double-write” penalty (associated with the filestore journal). Bluestore utilizes RocksDB, which stores object metadata, a write ahead log, Ceph omap data and allocator metadata.Download