Cassandra
  1. Cassandra
  2. CASSANDRA-3748

Range ghosts don't disappear as expected and accumulate

    Details

    • Type: Bug Bug
    • Status: Resolved
    • Priority: Major Major
    • Resolution: Not a Problem
    • Fix Version/s: None
    • Component/s: Core
    • Environment:

      Cassandra on Debian

      Description

      I have a problem where range ghosts are accumulating and cannot be removed by reducing GCSeconds and compacting.

      In our system, we have some cfs that represent "markets" where each row represents an item. Once an item is sold, it is removed from the market by passing its key to remove().

      The problem, which was hidden for some time by caching, is appearing on read. Every few seconds our system collates a random sample from each cf/market by choosing a random starting point:
      String startKey = RNG.nextUUID())

      and then loading a page range of rows, specifying the key range as:
      KeyRange keyRange = new KeyRange(pageSize);
      keyRange.setStart_key(startKey);
      keyRange.setEnd_key(maxKey);

      The returned rows are iterated over, and ghosts ignored. If insufficient rows are obtained, the process is repeated using the key of the last row as the starting key (or wrapping if necessary etc).

      When performance was lagging, we did a test and found that constructing a random sample of 40 items (rows) involved iterating over hundreds of thousands of ghost rows.

      Our first attempt to deal with this was to halve our GCGraceSeconds and then perform major compactions. However, this had no effect on the number of ghost rows being returned. Furthermore, on examination it seems clear that the number of ghost rows being created within GCSeconds window must be smaller than the number being returned. Thus looks like a bug.

      We are using Cassandra 1.0.3 with Sylain's patch from CASSANDRA-3510

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          • Assignee:
            Unassigned
            Reporter:
            Dominic Williams
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