Mongodb faulting not unsing all the available memory

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Esteban Feldman

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Nov 8, 2012, 12:14:57 PM11/8/12

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Hi, this is the mongostat for the mongo shard0 (have 3 shards)

insert query update delete getmore command flushes mapped vsize res faults locked db idx miss % qr|qw ar|aw netIn netOut conn time
2 687 3 0 3 133 0 45.9g 93.2g 3.04g 77 vizvid:7.9% 0 121|0 47|0 90k 99k 347 17:05:55
4 468 6 0 1 57 0 45.9g 93.2g 3.16g 77 vizvid:1.3% 0 162|0 5|0 53k 69k 347 17:05:56
8 112 9 0 1 14 0 45.9g 93.2g 3.08g 49 vizvid:10.6% 0 158|0 8|0 18k 21k 347 17:05:57
10 255 12 0 1 46 0 45.9g 93.2g 3.2g 83 vizvid:2.3% 0 165|0 6|0 36k 40k 347 17:05:58
7 393 8 0 1 84 0 45.9g 93.2g 3.18g 64 vizvid:9.1% 0 164|0 9|0 56k 63k 347 17:06:00
5 384 8 0 1 40 0 45.9g 93.2g 3.15g 51 vizvid:10.6% 0 174|0 1|0 44k 55k 347 17:06:01
12 551 15 0 1 89 0 45.9g 93.2g 3.04g 50 vizvid:14.0% 0 143|0 32|0 73k 79k 347 17:06:02
4 371 9 0 1 54 0 45.9g 93.2g 3g 55 vizvid:10.2% 0 174|0 1|0 46k 58k 347 17:06:03
3 780 4 0 1 75 0 45.9g 93.2g 3.08g 54 vizvid:3.4% 0 172|0 114|0 80k 107k 347 17:06:04
7 399 8 0 1 62 0 45.9g 93.2g 3.03g 40 vizvid:12.0% 0 154|0 15|0 52k 63k 347 17:06:05

as you can see it says that the res is 3.04g

and if I look at the available memory:

free -m

total used free shared buffers cached

Mem: 7954 7911 43 0 22 7058

-/+ buffers/cache: 830 7124

Swap: 1953 35 1917

is mostly cached

Using Ubuntu 12.04.1 LTS Server

Any clue?

Dwight Merriman

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Nov 8, 2012, 2:29:31 PM11/8/12

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if things are in the file system cache that are part of your database but the page hasn't been "touched" by mongod since its last startup, it won't get counted in mongod's resident bytes.

so that might mean those pages just haven't been used since then.

or, it could be that your readahead is pretty high and that is making things be read in via readahead that aren't hit before cache ejection: