Uploaded image for project: 'Core Server'
  1. Core Server
  2. SERVER-14506

special top chunk logic can move max chunk to a shard with incompatible tag

    XMLWordPrintable

    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 2.6.6, 2.7.5
    • Component/s: Sharding
    • Labels:
      None
    • Operating System:
      ALL
    • Backport Completed:

      Description

      Issue Status as of Aug 08, 2014

      ISSUE SUMMARY
      On tag-aware sharded clusters, when the balancer is active and the top/bottom chunk needs to be split, the new chunk may be temporarily moved to a shard with an incompatible tag thus temporarily violating tagging.

      This issue does not apply to sharded clusters using hashed shard keys.

      USER IMPACT
      Users with a heavy insert load may experience poor performance due to unneccessary migrations.

      WORKAROUNDS
      Users may consider adding a split point at an extremely high, unreachable shard key to prevent this behavior.

      AFFECTED VERSIONS
      MongoDB production releases up to 2.6.5 are affected by this issue.

      FIX VERSION
      The fix is included in the 2.6.6 production release.

      Original description

      In special logic for splitting top or bottom chunk https://github.com/mongodb/mongo/blob/master/src/mongo/s/d_split.cpp#L886 sets shouldMigrate to true.

      https://github.com/mongodb/mongo/blob/master/src/mongo/s/chunk.cpp#L466 goes along with the idea as long as balancing isn't disabled, and uses Shard::pick() to find the new target shard for the "hot" chunk.

      However, https://github.com/mongodb/mongo/blob/master/src/mongo/s/shard.cpp#L425 does not seem to take tags into account and ends up moving the chunk to a shard from which it must then immediately be moved in the next round (violating tagging in the meanwhile).

      This problem does not apply to hashed shard key since special logic is bypassed in that case.

        Attachments

          Issue Links

            Activity

              People

              • Votes:
                0 Vote for this issue
                Watchers:
                8 Start watching this issue

                Dates

                • Created:
                  Updated:
                  Resolved: