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I don't understand why the storage size is significantly larger on the secondary than on the primary (450 GB vs 375 GB).

  • hardware, operating system, file system and mongodb configurations are identical on primary and secondary
  • storage engine is wired tiger
  • each day, a new database is created in which historical data for this day is stored
  • the last 14 days (databases) are kept
  • the oldest database is deleted after the day
  • documents are never updated or deleted, they are only inserted and read
  • there were no primary/secondary switches in the past

As you can see in the collections stats, count and avgObjectSize are identical between primary and secondary but storageSize is different. Why is this so? Can it be fixed (how)?

offerHistory02:SECONDARY> db.offerHistory.stats()
{
    "ns" : "log-2021-03-27.offerHistory",
    "size" : NumberLong("1147404775489"),
    "count" : 1293199443,
    "avgObjSize" : 887,
    "storageSize" : 483053527040,
    "capped" : false,
    "wiredTiger" : {
        "metadata" : {
            "formatVersion" : 1
        },
        "creationString" : "access_pattern_hint=none,allocation_size=4KB,app_metadata=(formatVersion=1),assert=(commit_timestamp=none,read_timestamp=none),block_allocation=best,block_compressor=snappy,cache_resident=false,checksum=on,colgroups=,collator=,columns=,dictionary=0,encryption=(keyid=,name=),exclusive=false,extractor=,format=btree,huffman_key=,huffman_value=,ignore_in_memory_cache_size=false,immutable=false,internal_item_max=0,internal_key_max=0,internal_key_truncate=true,internal_page_max=4KB,key_format=q,key_gap=10,leaf_item_max=0,leaf_key_max=0,leaf_page_max=32KB,leaf_value_max=64MB,log=(enabled=true),lsm=(auto_throttle=true,bloom=true,bloom_bit_count=16,bloom_config=,bloom_hash_count=8,bloom_oldest=false,chunk_count_limit=0,chunk_max=5GB,chunk_size=10MB,merge_custom=(prefix=,start_generation=0,suffix=),merge_max=15,merge_min=0),memory_page_image_max=0,memory_page_max=10m,os_cache_dirty_max=0,os_cache_max=0,prefix_compression=false,prefix_compression_min=4,source=,split_deepen_min_child=0,split_deepen_per_child=0,split_pct=90,type=file,value_format=u",
        "type" : "file",
        "uri" : "statistics:table:collection-2583--5720839012155665746",
        "LSM" : {
            "bloom filter false positives" : 0,
            "bloom filter hits" : 0,
            "bloom filter misses" : 0,
            "bloom filter pages evicted from cache" : 0,
            "bloom filter pages read into cache" : 0,
            "bloom filters in the LSM tree" : 0,
            "chunks in the LSM tree" : 0,
            "highest merge generation in the LSM tree" : 0,
            "queries that could have benefited from a Bloom filter that did not exist" : 0,
            "sleep for LSM checkpoint throttle" : 0,
            "sleep for LSM merge throttle" : 0,
            "total size of bloom filters" : 0
        },
        "block-manager" : {
            "allocations requiring file extension" : 72040317,
            "blocks allocated" : 72069969,
            "blocks freed" : 32831,
            "checkpoint size" : 483052630016,
            "file allocation unit size" : 4096,
            "file bytes available for reuse" : 880640,
            "file magic number" : 120897,
            "file major version number" : 1,
            "file size in bytes" : 483053527040,
            "minor version number" : 0
        },
        "btree" : {
            "btree checkpoint generation" : 322002,
            "column-store fixed-size leaf pages" : 0,
            "column-store internal pages" : 0,
            "column-store variable-size RLE encoded values" : 0,
            "column-store variable-size deleted values" : 0,
            "column-store variable-size leaf pages" : 0,
            "fixed-record size" : 0,
            "maximum internal page key size" : 368,
            "maximum internal page size" : 4096,
            "maximum leaf page key size" : 2867,
            "maximum leaf page size" : 32768,
            "maximum leaf page value size" : 67108864,
            "maximum tree depth" : 5,
            "number of key/value pairs" : 0,
            "overflow pages" : 0,
            "pages rewritten by compaction" : 0,
            "row-store internal pages" : 0,
            "row-store leaf pages" : 0
        },
        "cache" : {
            "bytes currently in the cache" : 427430184,
            "bytes read into cache" : 1750413,
            "bytes written from cache" : NumberLong("1163389289271"),
            "checkpoint blocked page eviction" : 11018,
            "data source pages selected for eviction unable to be evicted" : 924534,
            "eviction walk passes of a file" : 10012883,
            "eviction walk target pages histogram - 0-9" : 2069657,
            "eviction walk target pages histogram - 10-31" : 886769,
            "eviction walk target pages histogram - 128 and higher" : 0,
            "eviction walk target pages histogram - 32-63" : 433587,
            "eviction walk target pages histogram - 64-128" : 6622870,
            "eviction walks abandoned" : 44203,
            "eviction walks gave up because they restarted their walk twice" : 1968,
            "eviction walks gave up because they saw too many pages and found no candidates" : 1804734,
            "eviction walks gave up because they saw too many pages and found too few candidates" : 14522,
            "eviction walks reached end of tree" : 1818021,
            "eviction walks started from root of tree" : 1866522,
            "eviction walks started from saved location in tree" : 8146361,
            "hazard pointer blocked page eviction" : 73210,
            "in-memory page passed criteria to be split" : 923037,
            "in-memory page splits" : 147932,
            "internal pages evicted" : 531175,
            "internal pages split during eviction" : 5277,
            "leaf pages split during eviction" : 170222,
            "modified pages evicted" : 184054588,
            "overflow pages read into cache" : 0,
            "page split during eviction deepened the tree" : 2,
            "page written requiring cache overflow records" : 0,
            "pages read into cache" : 137,
            "pages read into cache after truncate" : 1,
            "pages read into cache after truncate in prepare state" : 0,
            "pages read into cache requiring cache overflow entries" : 0,
            "pages requested from the cache" : 3732724262,
            "pages seen by eviction walk" : 992177213,
            "pages written from cache" : 72067093,
            "pages written requiring in-memory restoration" : 142958112,
            "tracked dirty bytes in the cache" : 0,
            "unmodified pages evicted" : 60247131
        },
        "cache_walk" : {
            "Average difference between current eviction generation when the page was last considered" : 0,
            "Average on-disk page image size seen" : 0,
            "Average time in cache for pages that have been visited by the eviction server" : 0,
            "Average time in cache for pages that have not been visited by the eviction server" : 0,
            "Clean pages currently in cache" : 0,
            "Current eviction generation" : 0,
            "Dirty pages currently in cache" : 0,
            "Entries in the root page" : 0,
            "Internal pages currently in cache" : 0,
            "Leaf pages currently in cache" : 0,
            "Maximum difference between current eviction generation when the page was last considered" : 0,
            "Maximum page size seen" : 0,
            "Minimum on-disk page image size seen" : 0,
            "Number of pages never visited by eviction server" : 0,
            "On-disk page image sizes smaller than a single allocation unit" : 0,
            "Pages created in memory and never written" : 0,
            "Pages currently queued for eviction" : 0,
            "Pages that could not be queued for eviction" : 0,
            "Refs skipped during cache traversal" : 0,
            "Size of the root page" : 0,
            "Total number of pages currently in cache" : 0
        },
        "compression" : {
            "compressed pages read" : 63,
            "compressed pages written" : 61677121,
            "page written failed to compress" : 32009,
            "page written was too small to compress" : 10311695,
            "raw compression call failed, additional data available" : 0,
            "raw compression call failed, no additional data available" : 0,
            "raw compression call succeeded" : 0
        },
        "cursor" : {
            "bulk-loaded cursor-insert calls" : 0,
            "close calls that result in cache" : 0,
            "create calls" : 61,
            "cursor operation restarted" : 176248823,
            "cursor-insert key and value bytes inserted" : 1087461754174,
            "cursor-remove key bytes removed" : 0,
            "cursor-update value bytes updated" : 0,
            "cursors reused from cache" : 211664692,
            "insert calls" : 1220619643,
            "modify calls" : 0,
            "next calls" : 397,
            "open cursor count" : 0,
            "prev calls" : 1,
            "remove calls" : 0,
            "reserve calls" : 0,
            "reset calls" : 539520296,
            "search calls" : 76,
            "search near calls" : 0,
            "truncate calls" : 0,
            "update calls" : 0
        },
        "reconciliation" : {
            "dictionary matches" : 0,
            "fast-path pages deleted" : 0,
            "internal page key bytes discarded using suffix compression" : 60661951,
            "internal page multi-block writes" : 28312,
            "internal-page overflow keys" : 0,
            "leaf page key bytes discarded using prefix compression" : 0,
            "leaf page multi-block writes" : 150396,
            "leaf-page overflow keys" : 0,
            "maximum blocks required for a page" : 1,
            "overflow values written" : 0,
            "page checksum matches" : 90452,
            "page reconciliation calls" : 226401610,
            "page reconciliation calls for eviction" : 176333404,
            "pages deleted" : 41475470
        },
        "session" : {
            "object compaction" : 0
        },
        "transaction" : {
            "update conflicts" : 0
        }
    },
    "nindexes" : 2,
    "totalIndexSize" : 59087273984,
    "indexSizes" : {
        "_id_" : 34540539904,
        "offerId_1" : 24546734080
    },
    "ok" : 1,
    "operationTime" : Timestamp(1617146426, 6416),
    "$gleStats" : {
        "lastOpTime" : Timestamp(0, 0),
        "electionId" : ObjectId("7fffffff000000000000000b")
    },
    "$configServerState" : {
        "opTime" : {
            "ts" : Timestamp(1617146426, 4484),
            "t" : NumberLong(23)
        }
    },
    "$clusterTime" : {
        "clusterTime" : Timestamp(1617146426, 6443),
        "signature" : {
            "hash" : BinData(0,"o3Sx+m0MnfXkEQkfdUHHJWnlRl8="),
            "keyId" : NumberLong("6899518352868248974")
        }
    }
}

Versus:

offerHistory02:PRIMARY> db.offerHistory.stats()
{
    "ns" : "log-2021-03-27.offerHistory",
    "size" : NumberLong("1147404775489"),
    "count" : 1293199443,
    "avgObjSize" : 887,
    "storageSize" : 403410546688,
    "capped" : false,
    "wiredTiger" : {
        "metadata" : {
            "formatVersion" : 1
        },
        "creationString" : "access_pattern_hint=none,allocation_size=4KB,app_metadata=(formatVersion=1),assert=(commit_timestamp=none,read_timestamp=none),block_allocation=best,block_compressor=snappy,cache_resident=false,checksum=on,colgroups=,collator=,columns=,dictionary=0,encryption=(keyid=,name=),exclusive=false,extractor=,format=btree,huffman_key=,huffman_value=,ignore_in_memory_cache_size=false,immutable=false,internal_item_max=0,internal_key_max=0,internal_key_truncate=true,internal_page_max=4KB,key_format=q,key_gap=10,leaf_item_max=0,leaf_key_max=0,leaf_page_max=32KB,leaf_value_max=64MB,log=(enabled=true),lsm=(auto_throttle=true,bloom=true,bloom_bit_count=16,bloom_config=,bloom_hash_count=8,bloom_oldest=false,chunk_count_limit=0,chunk_max=5GB,chunk_size=10MB,merge_custom=(prefix=,start_generation=0,suffix=),merge_max=15,merge_min=0),memory_page_image_max=0,memory_page_max=10m,os_cache_dirty_max=0,os_cache_max=0,prefix_compression=false,prefix_compression_min=4,source=,split_deepen_min_child=0,split_deepen_per_child=0,split_pct=90,type=file,value_format=u",
        "type" : "file",
        "uri" : "statistics:table:collection-2583--1868846566931849507",
        "LSM" : {
            "bloom filter false positives" : 0,
            "bloom filter hits" : 0,
            "bloom filter misses" : 0,
            "bloom filter pages evicted from cache" : 0,
            "bloom filter pages read into cache" : 0,
            "bloom filters in the LSM tree" : 0,
            "chunks in the LSM tree" : 0,
            "highest merge generation in the LSM tree" : 0,
            "queries that could have benefited from a Bloom filter that did not exist" : 0,
            "sleep for LSM checkpoint throttle" : 0,
            "sleep for LSM merge throttle" : 0,
            "total size of bloom filters" : 0
        },
        "block-manager" : {
            "allocations requiring file extension" : 47004346,
            "blocks allocated" : 47029067,
            "blocks freed" : 32112,
            "checkpoint size" : 403410251776,
            "file allocation unit size" : 4096,
            "file bytes available for reuse" : 278528,
            "file magic number" : 120897,
            "file major version number" : 1,
            "file size in bytes" : 403410546688,
            "minor version number" : 0
        },
        "btree" : {
            "btree checkpoint generation" : 329809,
            "column-store fixed-size leaf pages" : 0,
            "column-store internal pages" : 0,
            "column-store variable-size RLE encoded values" : 0,
            "column-store variable-size deleted values" : 0,
            "column-store variable-size leaf pages" : 0,
            "fixed-record size" : 0,
            "maximum internal page key size" : 368,
            "maximum internal page size" : 4096,
            "maximum leaf page key size" : 2867,
            "maximum leaf page size" : 32768,
            "maximum leaf page value size" : 67108864,
            "maximum tree depth" : 5,
            "number of key/value pairs" : 0,
            "overflow pages" : 0,
            "pages rewritten by compaction" : 0,
            "row-store internal pages" : 0,
            "row-store leaf pages" : 0
        },
        "cache" : {
            "bytes currently in the cache" : 519160231,
            "bytes read into cache" : 108549491,
            "bytes written from cache" : NumberLong("1161908962381"),
            "checkpoint blocked page eviction" : 4910,
            "data source pages selected for eviction unable to be evicted" : 241550,
            "eviction walk passes of a file" : 5058704,
            "eviction walk target pages histogram - 0-9" : 1441924,
            "eviction walk target pages histogram - 10-31" : 706198,
            "eviction walk target pages histogram - 128 and higher" : 0,
            "eviction walk target pages histogram - 32-63" : 324631,
            "eviction walk target pages histogram - 64-128" : 2585951,
            "eviction walks abandoned" : 62344,
            "eviction walks gave up because they restarted their walk twice" : 0,
            "eviction walks gave up because they saw too many pages and found no candidates" : 1249672,
            "eviction walks gave up because they saw too many pages and found too few candidates" : 23472,
            "eviction walks reached end of tree" : 1139902,
            "eviction walks started from root of tree" : 1335874,
            "eviction walks started from saved location in tree" : 3722830,
            "hazard pointer blocked page eviction" : 9346,
            "in-memory page passed criteria to be split" : 551341,
            "in-memory page splits" : 148613,
            "internal pages evicted" : 247462,
            "internal pages split during eviction" : 2425,
            "leaf pages split during eviction" : 167334,
            "modified pages evicted" : 57437477,
            "overflow pages read into cache" : 0,
            "page split during eviction deepened the tree" : 2,
            "page written requiring cache overflow records" : 0,
            "pages read into cache" : 8699,
            "pages read into cache after truncate" : 1,
            "pages read into cache after truncate in prepare state" : 0,
            "pages read into cache requiring cache overflow entries" : 0,
            "pages requested from the cache" : 3808973750,
            "pages seen by eviction walk" : 569860253,
            "pages written from cache" : 47026827,
            "pages written requiring in-memory restoration" : 50006230,
            "tracked dirty bytes in the cache" : 0,
            "unmodified pages evicted" : 39345995
        },
        "cache_walk" : {
            "Average difference between current eviction generation when the page was last considered" : 0,
            "Average on-disk page image size seen" : 0,
            "Average time in cache for pages that have been visited by the eviction server" : 0,
            "Average time in cache for pages that have not been visited by the eviction server" : 0,
            "Clean pages currently in cache" : 0,
            "Current eviction generation" : 0,
            "Dirty pages currently in cache" : 0,
            "Entries in the root page" : 0,
            "Internal pages currently in cache" : 0,
            "Leaf pages currently in cache" : 0,
            "Maximum difference between current eviction generation when the page was last considered" : 0,
            "Maximum page size seen" : 0,
            "Minimum on-disk page image size seen" : 0,
            "Number of pages never visited by eviction server" : 0,
            "On-disk page image sizes smaller than a single allocation unit" : 0,
            "Pages created in memory and never written" : 0,
            "Pages currently queued for eviction" : 0,
            "Pages that could not be queued for eviction" : 0,
            "Refs skipped during cache traversal" : 0,
            "Size of the root page" : 0,
            "Total number of pages currently in cache" : 0
        },
        "compression" : {
            "compressed pages read" : 3402,
            "compressed pages written" : 44862562,
            "page written failed to compress" : 3760,
            "page written was too small to compress" : 2135269,
            "raw compression call failed, additional data available" : 0,
            "raw compression call failed, no additional data available" : 0,
            "raw compression call succeeded" : 0
        },
        "cursor" : {
            "bulk-loaded cursor-insert calls" : 0,
            "close calls that result in cache" : 0,
            "create calls" : 376,
            "cursor operation restarted" : 8863521,
            "cursor-insert key and value bytes inserted" : NumberLong("1151926070153"),
            "cursor-remove key bytes removed" : 0,
            "cursor-update value bytes updated" : 0,
            "cursors reused from cache" : 133353675,
            "insert calls" : 1291054491,
            "modify calls" : 0,
            "next calls" : 1,
            "open cursor count" : 17699,
            "prev calls" : 1,
            "remove calls" : 0,
            "reserve calls" : 0,
            "reset calls" : 266569743,
            "search calls" : 4385,
            "search near calls" : 0,
            "truncate calls" : 0,
            "update calls" : 0
        },
        "reconciliation" : {
            "dictionary matches" : 0,
            "fast-path pages deleted" : 0,
            "internal page key bytes discarded using suffix compression" : 79075940,
            "internal page multi-block writes" : 25191,
            "internal-page overflow keys" : 0,
            "leaf page key bytes discarded using prefix compression" : 0,
            "leaf page multi-block writes" : 151115,
            "leaf-page overflow keys" : 0,
            "maximum blocks required for a page" : 1,
            "overflow values written" : 0,
            "page checksum matches" : 182737,
            "page reconciliation calls" : 64953107,
            "page reconciliation calls for eviction" : 56370626,
            "pages deleted" : 7290000
        },
        "session" : {
            "object compaction" : 0
        },
        "transaction" : {
            "update conflicts" : 0
        }
    },
    "nindexes" : 2,
    "totalIndexSize" : 57366028288,
    "indexSizes" : {
        "_id_" : 33265238016,
        "offerId_1" : 24100790272
    },
    "ok" : 1,
    "operationTime" : Timestamp(1617146426, 6443),
    "$gleStats" : {
        "lastOpTime" : Timestamp(0, 0),
        "electionId" : ObjectId("7fffffff000000000000000c")
    },
    "$configServerState" : {
        "opTime" : {
            "ts" : Timestamp(1617146426, 4501),
            "t" : NumberLong(23)
        }
    },
    "$clusterTime" : {
        "clusterTime" : Timestamp(1617146426, 6443),
        "signature" : {
            "hash" : BinData(0,"o3Sx+m0MnfXkEQkfdUHHJWnlRl8="),
            "keyId" : NumberLong("6899518352868248974")
        }
    }
}

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