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  1. Core Server
  2. SERVER-16715

Distribution of data with hashed shard key suddenly biased toward few shards

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Duplicate
    • Affects Version/s: 2.8.0-rc4
    • Fix Version/s: None
    • Component/s: Sharding
    • Labels:
      None
    • Backwards Compatibility:
      Fully Compatible
    • Operating System:
      ALL
    • Steps To Reproduce:
      Hide

      I've repeated the behavior consistently (once each on mmapv1 and wiredTiger) but only on a rather complicated setup of a 10 shard cluster deployed on EC2 using MMS Automation. And unfortunately, attempts with a simpler workload generator on a locally deployed cluster on OS X have been unable to repro.

      Show
      I've repeated the behavior consistently (once each on mmapv1 and wiredTiger) but only on a rather complicated setup of a 10 shard cluster deployed on EC2 using MMS Automation. And unfortunately, attempts with a simpler workload generator on a locally deployed cluster on OS X have been unable to repro.

      Description

      On 2.8.0-rc4, both mmapv1 and wiredTiger, I've observed a peculiar biasing of chunks toward to a seemingly random 1 or 2 shards out of the 10 total for new sharded collections.

      The workload of the application is single-threaded and roughly as follows:
      1.) programmatically creates a sharded database and a sharded collection
      2.) creates the {_id:"hashed"} index and a {_id:1} index.
      3.) inserts ~220k documents, each about ~2kB in size, with a string _id.
      4.) repeats from step #1, flipping back and forth between two databases, but always on a new sharded collection. Meaning when the workload completes, there's two sharded databases, each with 24 sharded collections on {_id:"hashed"}, each collection containing ~220k documents.

      Initially my the workload application starts out distributing chunks across all shards evenly as expected for each new sharded collection. However at some indeterminate point, when a new collection is created and sharded, it's as though 1 or 2 shards suddenly become "sinks" for a skewed majority (~80%) of all inserts. The other shards do receive some of the writes/chunks for the collection, but most are biased toward these 1 or 2 "select" shards.

      After the workload completes, the balancer does eventually redistribute all chunks evenly.

      I've had difficulty reproducing with a simpler setup, so I'm attaching some logs for the wiredTiger run where exactly 1 shard ("old_8") was the biased shard:

      • mongos.log.2015-01-03T21-22-44 - single mongos the workload was talking to
      • mongodb.primary-recipient.log.2015-01-03T21-22-31 - the primary of the suddenly "hot" biased shard "old_8"
      • mongodb.first-configsvr.log.2015-01-03T21-22-34 - first config server in the list of 3.

      Timing of logs:

      • on Jan 3rd about 18:06 UTC the run beings
      • on Jan 3rd about 19:55 UTC the shard old_8 primary "JM-c3x-wt-12.rrd-be.54a0a5b5e4b068b9df8d7b9c.mongodbdns.com:27001" begins receiving the lion's share of writes. (opcounter screenshot from MMS attached, as well as MMS chunks chart for an example biased collection.)
      • in general, it seems like collections after "benchmarks2015010500" from both databases are when the issue starts. E.g., "benchmark2015010500", "benchmark2015010501", "benchmark2015010502", etc

      Environment:

      • EC2 east, Ubuntu 14.04 c3.xlarge
      • All nodes on same VPC subnet
      • 2.8.0-rc4, 1GB oplog, journaling enabled
      • Logs attached are for wiredTiger, but have observed on both engines.

        Attachments

        1. Image 2015-01-04 at 3.57.31 PM.png
          Image 2015-01-04 at 3.57.31 PM.png
          26 kB
        2. mongodb.first-configsvr.log.2015-01-03T21-22-34.gz
          1.94 MB
        3. mongodb.primary-recipient.log.2015-01-03T21-22-31.gz
          1.60 MB
        4. mongos.log.2015-01-03T21-22-44.gz
          1.28 MB
        5. mongos.sh.status.out
          54 kB
        6. Screen Shot 2015-01-04 at 4.02.11 PM.png
          Screen Shot 2015-01-04 at 4.02.11 PM.png
          49 kB

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              Assignee:
              siyuan.zhou Siyuan Zhou
              Reporter:
              john.morales John Morales
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                Dates

                Created:
                Updated:
                Resolved: