MongoDB Journaling and the Storage Enginer
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The document describes MongoDB's directory layout and data structures. Key points: - MongoDB stores each database in its own set of preallocated files containing extents. - Data is memory mapped from files into virtual memory for fast access. - The journal logs all operations before writing to data files for durability. - Collections are divided into fixed-size extents containing records with BSON documents. - Indexes are also stored in extents containing index records that point to documents.
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MongoDB Journaling and the Storage Enginer
- 1. 1
- 2. Directory Layout • Separate files per database • Aggressive preallocation • Files contain one or more extents -rw------- 1 ben ben 64M May 1 19:14 test.0! -rw------- 1 ben ben 128M May 1 19:14 test.1! -rw------- 1 ben ben 256M May 1 18:25 test.2! -rw------- 1 ben ben 512M May 1 19:14 test.3! -rw------- 1 ben ben 1.0G May 1 19:14 test.4! -rw------- 1 ben ben 2.0G May 1 18:58 test.5! -rw------- 1 ben ben 16M May 1 19:14 test.ns! 2
- 3. Memory Mapping 0x7fffffffffff STACK! …! LIBS! …! test.ns! Disk test.0! test.1! …! ! …! HEAP! { … } MONGOD! NULL! 0x0 Document Process Virtual Memory
- 4. Data Structures • DiskLoc • Stores file number and offset of data on disk • Record *r = mmap base + DiskLoc.offset! • Max offset is 2^31 (2GB)! • NamespaceDetails • Stores collection metadata! • Extent! • Stores contiguous blocks within a namespace • Max extent size is 2GB • Record! • Holds a BSON document or B-tree bucket • DeletedRecord overwrites a Record! • Includes Padding
- 5. Namespace Details • Holds metadata about a collection or index • Stored in 1KB buckets in <dbname>.ns file • .ns file fixed size of 16MB • Maintains document count • Contains heads of linked lists NamespaceDetails firstExtent lastExtent _indexes[] stats freeList[]
- 6. Extent Structure Extent Extent length length xNext xNext xPrev xPrev firstRecord firstRecord lastRecord lastRecord
- 7. Extents > db.foo.validate( { full : true } ).extents.forEach( function(z){ print( z.loc + "tt" + z.size ); } ) 0:3000 20480 0:12000 81920 0:26000 327680 0:76000 1310720 0:1da000 5242880 0:76a000 6291456 0:d6a000 7553024 0:16de000 9064448 0:1f83000 10878976 0:29e3000 13058048 1:2000 15671296 1:ef4000 18808832 1:29e4000 22573056
- 8. Index Extents > db.system.namespaces.find() { "name" : "test.foo" } { "name" : "test.system.indexes" } { "name" : "test.foo.$_id_" } > db["foo.$_id_"].validate( { full : true } ).extents.forEach( function(z){ print( z.loc + "tt" + z.size ); } ) 0:9000 36864 0:1b6000 147456 0:6da000 589824 0:149e000 2359296 1:20e4000 9437184
- 9. Extents and Records Extent length xNext Data Record xPrev length Document { rNext firstRecord _id: “foo”, ... rPrev } lastRecord
- 10. Extents and Records Extent length xNext Data Record xPrev length Document { rNext firstRecord _id: “foo”, ... rPrev } lastRecord
- 11. Extents and Records Extent length xNext Data Record Data Record xPrev length Document length Document { { rNext rNext firstRecord _id: “foo”, _id: “foo”, ... ... rPrev } rPrev } lastRecord
- 12. BSON Format { hello: “world” } Doc Length Value Type x16x00x00x00 x02hellox00 ! x06x00x00x00 worldx00x00! Value Length
- 13. Index Extents Extent length Index Record Index Record xNext xPrev length Bucket length Bucket parent parent rNext rNext firstRecord numKeys numKeys rPrev K rPrev lastRecord { Document }
- 14. Index Extents 4 9 1 3 5 6 8 A B Extent length Index Record Index Record xNext xPrev length Bucket length Bucket parent parent rNext rNext firstRecord numKeys numKeys rPrev K rPrev lastRecord { Document }
- 15. Journaling • Write ahead logging • Operations written to journal before memory mapped regions • Private view • Shared view • Once journal written, data safe unless hardware problem • By default, journal flushed every 100ms, 100mb of writes, or on write concern of j=true • User configurable with --journalCommitInterval
- 16. Journal Format JHeader • Section contains single group commit JSectHeader [LSN 3] • Applied all-‐or-‐nothing DurOp DurOp DurOp Op_DbContext Set database context for JSectFooter length subsequent operations offset JSectHeader [LSN 7] fileNo DurOp data[length] DurOp length offset Write Operation DurOp fileNo data[length] JSectFooter length … offset fileNo data[length]
- 17. Journal Performance • On 99.9% read systems, no impact • Write performance degraded 5-30% when journal on same drive • Separate drive as low as 3%
- 18. Journal Admin • Journal stored in /dbpath/journal folder • If faster, three 1gb files may be preallocated • Can symlink to a different spindle • --journalCommitInterval* (2ms - 300ms) • When to journal • Single node: required for data integrity • Replica set: at least 1 node • All nodes: removes possible need to resync
- 19. Fragmentation • Files may become fragmented over time if documents change size • Free lists also contribute to fragmentation • 2.0 reduced scanning to reasonable amounts • 2.2 will change allocation strategy • Need to re-write free list to do online compaction
- 20. Compaction • 1.8 and previous: repairDatabase • 2.0+ : compact command • Currently resets paddingFactor, but can be changed. • Index (re)generation is now concurrent, so compaction can be N times faster • Generally causes some extra allocation • Does not delete or truncate files
- 21. Planned Changes • Split data and indexes into different files • Indexes could by symlinked to a different drive (SSD) • Improved allocation strategy
- 22. Download MongoDB http://www.mongodb.org/downloads Ben Becker ben.becker@10gen.com