M202: MongoDB Advanced Deployment and Operations

README.md

https://university.mongodb.com/courses/M202/about

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• Memory model
  • mmap (deprecated after introducing WiredTiger engine)
• Working set (portion of data that is accessed most often)
  • indexes
  • subset of data
• Resident memory (amount of RAM currently used by the database process)
  • shared view (mmap of data file) vs. private view (remap of shared view)
    • remapping doubles usage of virtual memory usage (not physical)
    • private view is used for journalling
    • sudden drop of resident memory (when part of memory is marked as filesystem cache)
      • data is still cached in memory
• Wired Tiger
  • document level locking
  • compression (zlib, snappy)
  • half of available memory is used for WT cache
• Process restarts
  • after killing mongod, some data is still in memory (FS cache)
  • to drop FS caches
    • echo 3 > /proc/sys/vm/drop_caches
    • sysctl -w vm.drop_caches=1
• Storage
  • spinning disks
    • capped collections
    • journal
• File systems & options
  • ext4 & xfs support fallocate() (which is fast)
  • ext3 does not support fallocate() and requires filling file with zeroes to allocate 2G file
    • this is slow
• Readahead
  • number of extra sectors to be read form disk on data access
  • # blockdev --report
  • big readahead is good when
    • data locality is high
    • data seeks are expensive (not so true for SSD)
  • lower readahead is preferred when effective memory usage is required (prevent thrashing FS cache)
    • lower boundary for readahead is equal to index bucket size (8k = 16 sectos)